3. Genus—TETRACLITA. Pl. 10, 11.

3. Genus—TETRACLITA. Pl. 10, 11.

TETRACLITA. Schumacher. Essai d’un Nouveau Syst., &c., 1817.

CONIA. Leach. Journal de Physique, tom. 85, 1817.[105]

ASEMUS. Ranzani. Memoire di Storia Naturale, 1820.[106]

POLYTREMA. De Ferussac. Dict. Classique d’Histoire Naturelle, 1822.

LEPAS. Gmelin. Systema Naturæ, 1789.

BALANUS. Bruguière. Encyclop. Method., 1789.

------ Lamarck. Animaux sans Vertèbres, 1818.

Compartments four, sometimes externally calcified together: parietes permeated by pores, generally forming several rows. Basis flat, irregular, calcareous, or membranous.

Hab.—Throughout the tropical and warmer temperate seas.

General Appearance.—The shell is conical, more or less depressed, and very rarely, even when growing in crowded groups, becomes cylindrical or elongated. The orifice is seldom large, generally diamond-shaped or oval. The colour is either nearly white or purple, occasionally even inky black, or very dark green, and sometimes of a pale pink peach-blossom. The surface is sometimes smooth, but more commonly longitudinally ribbed; the outer lamina of shell is very often wholly corroded away, excepting close to the basis, leaving the solidly upfilled parietal tubes exposed: these give the shell a striated appearance, or they appear like flattened tapering points adpressed to its surface (Pl. 10, fig. 1 b): Lamarck attempted to express this appearance, by using the specific name of stalactiferus. The colour of the shell depends, to a considerable extent, on the colour of the shelly matter in these exposed parietal tubes. We shall presently see that the corrosion and disintegration, to which some of the species are so liable, plays an important part during their growth. The radii are either well developed, as in most of the species; or they are entirely absent, as in the great majority of specimens of T. porosa and serrata. In many individuals of T. porosa and purpurascens not only are the radii absent, but the four compartments are calcified together without any trace, on the external surface, of the sutures. The largest specimen which I have seen of T. porosa, which is the largest species, was two inches in basal diameter, and nearly one inch and a half in height.

Scuta.—These valves are sub-triangular, and either longitudinally or transversely elongated. Externally, the growth-ridges are moderately prominent, and in T. costata and cœrulescens they are crossed by longitudinal striæ. Along the occludent margin, the inflected extremity of each alternate growth-ridge is generally much thickened,—a set of teeth being thus formed, by which the two valves are locked together. In T. porosa, this character is variable, for sometimes every alternate ridge, and sometimes only two or three ridges, separated from each other by several growth-ridges, are thus developed into teeth. The articular ridge is either moderately prominent, or is extremely prominent, as in T. cœrulescens; but the lower edge in no case depends as a free, hinge-like style, as sometimes in Balanus. The adductor ridge is generally well developed and distinct from the articular ridge: in T. purpurascens it is very blunt: in T. serrata it is united to the articular ridge half way up it, thus forming a deep tubular cavity running up to the apex of the valve: in T. cœrulescens, the adductor ridge is very short, and is united to, or almost continuous with, the lower end of the articular ridge, a small sub-cylindrical tubular cavity being thus formed. Small crests exist for the attachment of the rostral and lateral depressor muscles, in most of the species, excepting T. purpurascens and costata, in which, however, more especially in the former, there are, instead of crests, minute pits for the attachment. These crests vary much in prominence in the same species.

Terga.—These valves present no essential differences from those of Balanus; they are sometimes beaked, and the beak is hollow and occupied by a thread of corium, as in that genus. The external surface of the valve is often depressed in the line of the spur, but there is never a longitudinal furrow with the edges folded in, as in Balanus. The spur is very short in T. purpurascens. In T. radiata, the articular ridge is remarkably prominent. The crests for the depressor muscles are well developed in all the species. The shape of the terga is variable in nearly all the species, and greatly so in T. porosa.

Compartments.—Owing to there being only four compartments, the lateral pair are large; the size of the carina relatively to the rostrum varies, according as its alæ have been added to during diametric growth. The walls are very thick, and are composed of numerous tubes, in some species as many as fourteen or fifteen rows being exposed on the basal margin (Pl. 10, fig. 1 g). The tubes are generally angular, and slightly elongated in the ray of the circular shell; sometimes they are nearly circular and small. New tubes are formed only at the basal edge of the outer lamina, by the bifurcation of the septa which form the tubes. In very young specimens there is only a single row of tubes; and in T. rosea this holds good throughout life: in this species (fig. 3 d) the tubes, in the single row, are large and quadrangular, and the outer lamina of shell is strengthened by numerous, small, internal, longitudinal plates. I believe the branching septa, which separate and form the parietal tubes, correspond with the longitudinal septa in the more simple walls of Balanus. The tubes become solidly filled up, in their upper parts, with hard, and generally coloured shelly matter. The degree to which they are filled up differs in the different species; the external side of each tube is always first thus coated. The thin outer lamina of shell, in several of the species, commonly disintegrates and disappears; the upfilled parietal tubes being thus exposed. The inner lamina of the walls is generally smooth, but in T. radiata it is longitudinally ribbed, as in most species of Balanus. The sheath is generally dark-coloured; its lower edge does not project or overhang the inner lamina, as is usual in Balanus, excepting in T. serrata, and in some few varieties of T. porosa.

The Radii, when developed, are either narrow or broad, with their summits either oblique or extending in a straight line from the top of one compartment to that of another. In T. serrata, I have not seen a single specimen with the radii developed; in T. porosa, they are very seldom developed, and then, apparently, only in quite young specimens, in which they are narrow; in T. purpurascens, they seem to be about as often developed as not, and when present they are broad; in T. costata, cœrulescens, and radiata, they are always largely developed. In some specimens of the species, in which the radii are not developed, even the sutures do not reach the external surface; the outer lamina of the parietes being continuous all round, so that the shell seems formed of a single piece. Even in such specimens, the four compartments, viewed from within, can be seen to be distinct; and the sutures can generally be traced across the whole thickness of the parietal tubes; in this latter case, when the sutures are broken open, the radii are seen to be represented (Pl. 10, 1 h) by a few small sinuous ridges. Owing to the disintegration of the upper and outer part of the shell, and the consequent exposure of parts of the sheath and alæ, the radii sometimes appear as if developed, when such is not the case. With respect to the internal structure of the radii, they are formed, in T. purpurascens and costata, of tubes, like those of the parietes, and therefore according to the normal plan; whilst in the other species they are formed by longitudinal sinuous ridges, sending out on each side irregular denticuli; and the interspaces between the ridges are filled up solidly during the growth of the radii, in all the species, except in T. radiata, in which they are left to a considerable extent open. These sinuous ridges, with their denticuli, homologically represent the branching septa which form the parietal tubes. The edges of the alæ are crenated in all the species, except in T. costata.

Diametric growth.—When first examining groups of T. porosa, in none of which the radii had been developed, and in which, consequently, the shell could not have grown in diameter, but only at its basal margin, I was at first unable to comprehend, how the upper part of the shell and the orifice could have acquired their proper proportional width. The young shell, at its first formation, starts with an orifice so small that a pin could hardly be inserted in it; and this in many individuals is never increased in diameter by the diametric growth of the shell; but in place of this, as the conical shell is added to at its base, the whole upper part disintegrates and wears away, the orifice becoming thus enlarged. We thus see that the corrosion and wearing away of the upper part of the shell is a necessary element in its growth. The development of the radii, which in some of the species, as in T. purpurascens, at first seems to be quite capricious, really depends upon the fact, whether the specimens have been exposed to disintegration; for I have almost always found that when the outer lamina of shell has been well preserved, the radii have been developed, and the orifice has been enlarged by their growth, instead of by the wearing down of the upper part of the conical shell.

Basis.—This consists of a very thin, flat, though irregular, translucent, calcareous plate, which towards the edges is sometimes membranous. In T. purpurascens, the basis is entirely membranous. When a portion of the calcareous base is dissolved in acid, a tissue is left, composed of several laminæ, to which numerous bifurcating cement-ducts are attached: even before dissolution, these delicate bifurcating ducts can just be perceived by the aid of a simple lens.

Mouth.—The several organs present no particular characters. There are generally three teeth on each side of the notch in the labrum. The palpi usually have parallel sides, but are club-shaped in T. purpurascens and costata. The mandibles have generally four teeth, but there are five in T. vitiata, and only three in T. costata. The maxillæ are notched. The outer maxillæ are bilobed in front.

Cirri.—The segments of the three posterior pairs usually support only three pairs of main spines, but there are four pairs in T. vitiata and costata: between each pair, there is either a tuft of fine spines, or a single fine spine. The rami of the first cirrus are unequal in length. In the third cirrus, the posterior ramus is sometimes much elongated, but sometimes both rami are short and blunt. Some of the segments in the third cirrus often support very coarsely and doubly pectinated spines. Under the head of T. porosa, it will be seen to what a remarkable degree the relative numbers of the segments in the several cirri vary, even in specimens taken out of the same cluster.

The Branchiæ are well developed (at least in T. porosa and costata), as a large, plicated, tapering fold, with a small second fold on the inner side at the base. In T. porosa the stomach is destitute of cæca. The vesiculæ seminales in this same species are large, with their broad, blunt ends reflexed. The ovarian tubes surround the sack, and cover the basal plate.

Distribution and Habitats.—This genus is confined to the tropics, and to the warmer parts of the temperate seas: in the southern hemisphere, it ranges south, to the Cape of Good Hope and to Van Diemen’s Land: in the northern hemisphere it does not appear to range so far; I do not know of any authentic case of a species having been found in the Mediterranean, or on the shores of the United States, north of the West Indies. Tetraclita porosa is found round the whole world; T. radiata, also, has a very wide range, inhabiting the West Indies, the East Indian Archipelago, and New South Wales. This latter species, as well as T. cœrulescens, is often attached to the bottoms of ships, adhering to Balanus tintinnabulum. The several species live attached to tidal rocks, to littoral shells, or to massive corals. I have met with two instances, in the West Indies and the Philippine Archipelago, of T. porosa adhering to wood. Tetraclita porosa seems to feed chiefly on crustaceans: the number and the size of the amphipods, isopods, and entomostracans, together with an annelid, in the stomachs of some specimens from South America, was quite surprising. As many as five species occur in the same region, in the eastern half of the world; thus on the shores of New South Wales, we have T. porosa, vitiata, radiata, purpurascens, and rosea; in the Philippine Archipelago, we have T. porosa, vitiata, costata, and cœrulescens.

I have not seen any species of this genus fossil.

Variation.—The species vary in shape nearly, but not quite as much as in Balanus, for we very rarely here see cylindrical varieties; but in the same species, we have extremely depressed, steeply conical, and convex forms; the orifice varies in relative size; the state of the surface, whether ribbed or smooth, whether well preserved or corroded, the upfilled parietal tubes being thus exposed, varies more than in Balanus. The colour also varies; specimens of T. porosa (Pl. 10, fig. 1 a to 1 f) being dark purple, or even inky black, white, pale pinkish-purple, and green: as far as I have seen, the pinkish varieties of T. porosa are confined to the eastern half of the world. Even the ridges and crests on the under side of the scutum vary in some degree; and the tergum in T. porosa varies considerably, and in some of the other species, slightly, in general shape: I believe that the tergum becomes narrow and elongated, when the shell is steeply conical, with the orifice of small diameter. After having spent several days in disarticulating and closely examining the many specimens of T. porosa in my possession, I concluded that its varieties formed at least four species, these being in external appearance extremely distinct; but ultimately the many intermediate forms compelled me to unite all into a single species. Again, I may instance the conical, ribbed variety of T. purpurascens (Pl. 11, fig. 1 b), with the outer lamina of the shell preserved and with the radii widely developed, as having not the smallest resemblance to the other common depressed variety (fig. 1 a), having a granulated surface, produced by the exposed tips of the upfilled parietal tubes, and without a trace of the radii or even of the sutures; I may add that, according to the characters used by some authors, these two varieties would be classed in two separate genera.

To distinguish the species of this genus, the chief reliance must be placed (as in the case of most other sessile cirripedes) on the general outline of the opercular valves, and on the ridges and crests on their under sides: the internal structure, however, of the radii, and in a lesser degree of the parietes, must not be overlooked. I have not found the parts of the mouth, or the differences in the cirri, of much service; and it will be shown under T. porosa, that the relative numbers of the segments in the several cirri, and even their shape, is extraordinarily variable.

Affinities.—All the species are pretty closely allied, and there is no ground for making any sectional division of the eight species, more especially not on the ground whether or no the radii are developed. Tetraclita purpurascens, taking into account all the characters, including the animal’s body, is perhaps the most distinct species in the genus, but it is closely allied to T. costata. In the well-marked character of the parietes being formed of a single row of large tubes, T. rosea differs from all the other species. The genus is closely allied to Balanus; I can point out no difference in the animal’s body, nor any constant difference in the opercular valves. The ridges and crests on the under sides of the scuta are more prominent in Tetraclita; and all the species, excepting two, have crests (though sometimes very slight) for the attachment of the rostral depressor muscle, and these occur only in two species of Balanus: crests, also, for the attachment of the lateral depressor muscle are common in Tetraclita, but rare in Balanus. The main character, however, of the genus, as compared with Balanus, is derived from the existence of only four compartments, and in a lesser degree from the several rows of parietal pores; but, as just stated, T. rosea has only a single row, and, on the other hand, in Balanus crenatus, there is a slight tendency exhibited, in the dividing of the longitudinal septa near the outer lamina, to form a second row of parietal pores; and in B. cariosus, moreover, we actually have two or three rows of irregular and very short pores. The thin, yet solid calcareous basis which occurs in all the species of Tetraclita, excepting T. purpurascens, resembles the basis in Balanus flosculus and imperator, but I suspect that the structure of the cement-ducts is different in the two genera.

1. TETRACLITA POROSA. Pl. 10, fig. 1 a-1 m.

LEPAS POROSA. Gmelin. Syst. Naturæ, 1789.[107]

BALANUS SQUAMOSUS. Bruguière. Encyclop. Method., 1789, Pl. 165, fig. 9, 10.

LEPAS FUNGITES. Spengler. Skrivter af Naturhist. Selskabet, 1 B., 1790.

---- POROSA. Wood’s General Conchology, Pl. 9, fig. 4, 1815.

TETRACLITA SQUAMULOSA. Schumacher. Essai d’un Nouveau Syst., &c., 1817.

BALANUS STALACTIFERUS. Lamarck. Animaux sans Vertèbres, 1818.

------ ------------ Chenu. Illust. Conch., Pl. 4, fig. 6, 7.

ASEMUS POROSUS. Ranzani. Memoire di Storia Naturale, Tab. 3, fig. 32-35.

CONIA POROSA. Sowerby. Genera of Recent and Fossil Shells, Plate, 1823.

---- ---- Leach. (sine descript.) Encyclop. Brit. Supplement, vol. 3, 1824, Tab. 57.

Radii rarely present, when present narrow; even the sutures often absent: shell steeply conical, with the surface generally corroded, and having a stalactiferous appearance.

Var. (1) communis (Pl. 10, fig. 1 a): outer lamina of shell almost wholly removed; the portion preserved, as well as the exposed parietal tubes, gray, or pale dirty brown, or dull purple.

Var. (2) nigrescens (Pl. 10, fig. 1 c): outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, very dark purple or inky black.

Var. (3) viridis: outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, green; under surface of opercular valves clouded green.

Var. (4) rubescens (Pl. 10, fig. 1 b): outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, pale reddish-purple, or peach-blossom purple; under surfaces of opercular valves often reddish purple; terga often rather narrow, with the spur somewhat pointed, and with the basal margin on the carinal side sloping, or not making an angle with the spur.

Var. (5) elegans (Pl. 10, fig. 1 d): outer lamina preserved, excepting sometimes near the summit of the shell; white, tinged with yellowish brown from the epidermis; surface strongly ribbed longitudinally; orifice rather small; sheath reddish-purple; terga narrow, with the basal margin sloping as in var. rubescens.

Var. (6) communis (young) (Pl. 10, fig. 1 e): radii developed, very narrow; outer lamina of shell preserved, gray or dull purple; surface slightly ribbed longitudinally.

Var. (7) patellaris (Pl. 10, fig. 1 f): radii developed, very narrow, white; outer lamina of shell preserved, generally reddish-purple; steeply conical, with the orifice extremely small; surface smooth, with longitudinal white ribs. Terga very narrow, with the spur sharply pointed, and with the basal margin on the carinal side sloping towards it, or not making an angle with it. Scuta, with the adductor ridge very prominent. Attached to a ship’s bottom.

Hab.—West Indies, Brazil, West Colombia, Panama, Galapagos Archipelago, California, Philippine Archipelago, China, East coast of Australia, Red Sea; generally attached to tidal rocks, sometimes to shells, sometimes to wooden posts. Very common.

General Appearance.—This, the widest-distributed and much the commonest species of the genus, varies greatly in external appearance. The usual shape is steeply conical, but some individuals are much depressed. In the common varieties the outer lamina of shell has been removed even close to the basal edge; the upfilled parietal tubes being thus exposed (fig. 1 b), as flattened adpressed points. These points are largest in large specimens, but they vary somewhat in size in specimens of equal growth. When the outer surface is preserved, it is generally ribbed longitudinally, but is sometimes quite smooth. The most general colour is dirty gray or dark purple; but many specimens are pale pinkish-purple, owing to the exposure of the parietal tubes upfilled with shelly matter of this tint: there are also, as given under the characters of the vars., black, white and green varieties. The sheath is always tinted by the prevailing colour. The radii are rarely developed, but generally the four sutures are distinct; sometimes these are externally quite obliterated, the shell, as seen from the outside, consisting of a single piece like a patella or fissurella. When the radii are developed they are very narrow, with their upper edges oblique: their development seems always coincident with the more or less perfect preservation of the shell, and their function is to enlarge the orifice; the enlargement being usually effected by the disintegration and removal of the whole upper part of the conical shell. The size of the orifice varies considerably; in the seventh variety it was extraordinarily small: in outline it varies from oval to rounded trigonal or rhomboidal; in some specimens, with the radii well developed, it was rounded pentagonal.

Size.—Tetraclita porosa is the largest species of the genus; I have seen specimens attached to a large pebble of granite in the British Museum, which measured two inches in basal diameter, and nearly one inch and a half in height.

Scuta: these are sub-triangular and generally a little elongated, but they vary slightly in relative breadth (fig. 1 i, 1 l), and likewise in the degree to which the basi-tergal angle is rounded off. The under surface is clouded with dull or pinkish-purple, or with green, or is nearly white. The articular ridge is not prominent, and the articular furrow is narrow. The adductor ridge is prominent, and runs upwards for some distance close and parallel to the articular ridge; and sometimes it extends nearly or quite up to the apex of the valve; in one single specimen the adductor ridge had an abraded appearance, and was very little developed. The crests for the rostral and lateral depressores are sharp and distinct. Along the occludent margin, the ends generally of the alternate lines of growth are enlarged into knobs, serving to lock the two valves together; but in many specimens only two or three knobs, at intervals of several lines of growth (fig. 1 b), are developed.

Terga: when the upper end of the valve is not corroded, there is a distinct beak, hollow within for a thread of corium. The scutal margin is not much inflected, and the articular ridge not very prominent. The spur is placed quite close to the basi-scutal angle of the valve, so that there is no basal margin on that side of the valve. The width of the valve and of the spur, and the acumination of the extremity of the latter, varies in a remarkable manner. In the broad and commonest variety (fig. 1 k), the basal margin of the valve form an angle of about 130° with the carinal side of the spur, and the basal end of the spur is broad and truncated. In the less common and narrow variety (1 m), the basal margin in some extreme cases forms very nearly a straight line with the carinal side of the spur; and the spur itself is bluntly pointed: in var. 7 it is sharply pointed.

Structure of the parietes and radii.—In all cases the four sutures are quite distinct, from top to bottom, on the internal lamina of the shell, and generally they run through the whole thickness of the walls, and are visible externally. Often they do not quite reach the outer lamina, and then the shell externally consists of a single piece, like a patella. Sometimes the sutures can be traced running through the parietal tubes only for a short distance from the internal surface; where they cease, the two walls of the suture become fused together. When a perfect suture is split open, the radius is represented (fig. 1 h) by a few narrow, sinuous ridges, sending out on each side little branches or denticuli; these are received into corresponding furrows in the opposed compartment. These ridges run nearly parallel to each other, and somewhat obliquely, from the outer lamina of the shell to the basis. When the radii are developed, their edges are similarly formed, by sinuous denticulated ridges, with the interspaces between them filled up solidly. The alæ are but little prominent.

The mouth does not deviate from the generic type. The cirri are remarkable from the variability in the several pairs of the relative numbers of their segments, as shown in the following table. The segments do not correspond even on opposite sides of the same individual, as may be seen in the two lower lines of the table. I believe that variability to this degree is very uncommon in other cirripedes, though, as stated in the Introduction, the number of the segments always increases with the growth of the individual. The terminal segments in the longer rami of both the first and third cirrus are antenniformed,—being elongated, and of a different shape, with fewer bristles, compared with the basal segments of the same cirri. It is apparently these terminal segments which are particularly liable to vary in number. In both rami of the third cirrus, some of the segments, from the sixth to the eleventh inclusive, (counting from the bottom), more especially the eighth, ninth, and tenth, carry a few spines coarsely and doubly pectinated; but as some of the adjoining segments carry spines which may be called doubly serrated, it is not easy to draw an exact line of demarcation. Sometimes, though rarely, a few of the nearly terminal segments in the second cirrus are furnished with similar, doubly pectinated spines.

Numbers of the segments in the rami of the Cirri, in different specimens.

First cirrus. Second cirrus. Third cirrus. Sixth cirrus.

Shorter ramus. Longer ramus. Shorter ramus. Longer ramus. Shorter ramus. Longer ramus. Either ramus.

Specimen from Pernambuco, Brazil (var. 1) 9191011152726

Another specimen from the same cluster (var. 1) 12231214141923

From the Galapagos Archipelago, (var. 1) 10161011? 11 1625

From the Philippine Archipelago, (var. 4) 10? 10 10 11 19 18

From California (var. 5) 15 24———— 22

From the bottom of a ship (var. 7), young —— 9 9 10 17 14

From South America (var. 1) British Museum 11 18 11 12 13 21 22

Another specimen from the the last specimen 16 25 13 18 15 23 27

Same individual, but the cirri from the opposite side of the body 15 28 15 21 16 23—

Varieties.—Under the generic description, I have stated that after having spent some time in examining a very large suite of specimens of T. porosa, I concluded that at least four of the varieties were true species. It so happened that all the specimens which I first examined of the var. (4) rubescens, had the narrow sloping terga, and scuta with only two or three great teeth on their occludent margins; but ultimately, in a group thus characterised, I found one or two individuals with terga precisely of the shape of those in var. (1) communis. Again, in a group of dull purple specimens of var. communis, a few had the narrow sloping terga, and scuta with teeth on their occludent margins, intermediate in size and number between the varieties with only one or two great teeth, and those with every alternate growth-ridge enlarged into a tooth. Hence var. rubescens completely broke down as a species.

With respect to var. (5) elegans (a M.S. specific name of Leach) I inferred at first, from external appearance alone, that it was distinct; the outer lamina of the shell and even the epidermis is preserved; the surface is strongly ribbed, and the whole shell, excepting the sheath, is nearly white; the terga are narrow, with a sloping basal margin as in var. (4) rubescens. Whole groups of specimens are thus characterised. But as var. communis is often white, and as the surface, when the outer lamina is preserved, is generally, as we shall presently see, ribbed longitudinally, the differences in var. elegans are quite unimportant.

The (6th) var. differs from var. communis only in the narrow radii having been developed, and consequently in the orifice being pentagonal, and in the outer, longitudinally ribbed lamina of the shell having been preserved. In the same group of specimens, I have seen every intermediate stage between this and the common form. It must not, however, be supposed that the young of var. communis always pass through these stages, for such is not the case. In the genus Balanus it has been seen, how capricious in some species is the development of the radii.

Of the other varieties, enumerated at the beginning, no further mention is required, excepting with respect to var. (7), the most singular of all. I have seen only three specimens, sent to me by Dr. Aug. Gould, of Boston, United States, and these from the appearance of their bases I have no doubt had been attached to a ship,—the only instance which I have met with, in the present species. The shell is steeply conical, with the orifice so small as to be reduced to a mere pore; the radii are extremely narrow and white; the shell is thin, with the surface smooth, but ribbed longitudinally and regularly; the outer lamina of the shell and the epidermis are perfectly preserved; the upper part of the shell is reddish purple, which dies away towards the base: careful examination of the apex shows that at the first growth the young shell was blueish-green. The terga are narrow, with a sloping basal margin, as in var. rubescens, but with the point of the spur sharper. I have formerly remarked that the shape of the terga seems influenced by the size of the orifice. The lower edge of the sheath depends freely: I have seen no other instance of this latter structure, so common, but so variable, in Balanus, in the present species, except to a partial extent in one distorted specimen, in Mr. Stutchbury’s collection, adhering to Balanus tintinnabulum, and probably taken from a ship’s bottom. I may add that this distorted specimen was remarkable from its radii being wider than in any other instance,—from its smooth uncoloured surface without longitudinal ribs,—and from the perfect preservation of the epidermis over its entire surface. Although Dr. Gould’s specimens, in external aspect, are absolutely and entirely different from the common varieties of T. porosa, there are so many intermediate forms, and the differences are so little important that I feel no hesitation in attributing them to variation, consequent on the individuals having been exposed to unusual conditions, attached to the bottom of a ship.

2. TETRACLITA SERRATA. Pl. 10, fig. 2 a-2 d.

Shell dark greenish-gray, with narrow, longitudinal, serrated ribs: radii absent: scutum with the adductor and articular ridges forming a cavity, which runs up to the apex of the valve.

Hab.—Cape of Good Hope; Algoa Bay; attached to sandstone and to Patellæ; Mus. Brit., Cuming, and Stutchbury.[108]

General Appearance.—Colour dark greenish gray; form steeply conical; surface covered, especially in the lower half of the shell, by numerous, narrow, sharp, longitudinal ridges, but slightly prominent, and serrated or transversely divided into small teeth: when the shell has been much disintegrated, the upper part of the surface consists of the exposed, smooth, rather large, upfilled parietal tubes. I have seen no instance of the development of the radii; sometimes even the sutures are with great difficulty distinguishable, though I believe they always reach the outer surface; sometimes the sutures are wide from the disintegration of the edges of the compartments. Orifice rounded or oval.

Scuta.—The scuta and carinal half of the terga are blueish-green. In the scuta neither the articular ridge or furrow are much developed: the adductor ridge is prominent, and is united to the articular ridge, about half way up the latter, thus forming a rather large, triangular cavity, which runs up to the apex of the valve.

The Terga are beaked. The spur, measured across the upper part, is half as wide as the valve; it is bluntly pointed; it is placed quite close to the basi-scutal angle of the valve, so that there is no basal margin on that side; it curves towards the scutum, its extremity extending beyond the basi-scutal angle.

Structure of the Shell and Radii.—The parietal tubes are rather large, especially those adjoining the inner lamina of the walls. The shell is of singularly little specific gravity, which is due to the parietal tubes not being filled up with shelly matter nearly to so great an extent as in the other species; even in the uppermost part the tubes are not solidly filled up, only their external sides are thickly coated with greenish-black shell, which by corrosion becomes grayish. The radii, as stated, are not developed: the shell breaks with singular facility along the sutures, and the radii are then seen to be most feebly represented by a few very small branching ridges. The alæ have their edges plainly crenated. The sheath is dark green, with the lower edge free.

The Mouth presents no particular characters. With regard to the Cirri, I am doubtful whether any confidence can be placed in the numbers of the segments being constant; but I may state that the second cirrus contained thirteen and sixteen segments in its two rami; the third cirrus only fourteen in both rami; and the sixth cirrus twenty-six segments in both rami. Whereas in every specimen of T. porosa, the longer ramus of the third cirrus contained more segments than either ramus of the second. About half the segments, namely, those in the middle of both rami of the third cirrus, are furnished with coarsely and doubly pectinated spines, like those in T. porosa.

Affinities.—Upon the whole, this species is more nearly allied to T. porosa than to any other. In the cavity formed by the union of the adductor and articular ridges, it is allied to T. cœrulescens. This species differs from all, in its little specific gravity, consequent on the parietal tubes being only slightly filled up, and in the peculiarly serrated, narrow, approximate ridges on the external surface of the walls. The character derived from the adductor ridge, just alluded to, is remarkable. In the shape of the terga, in the absence of radii, and in the structure of the body, this species approaches closely to T. porosa.

3. TETRACLITA ROSEA. Pl. 10, fig. 3 a-3 d.

CONIA ROSEA.[109] Krauss (!). Die Südafrikanischen Mollusken, Tab. 6, fig. 28, 1848.

BALANUS CUMINGII. Chenu. Illust. Conch., Tab. 4, fig. 5.

Shell dirty white, tinged with pink; parietes formed by a single row of large tubes: radii generally narrow: tergum with the spur rather short and broad.

Hab.—New South Wales, Moreton Bay in lat. 27°, Port Jackson, and Twofold Bay; South Africa, Algoa Bay. Attached, in Australia, to littoral rocks and shells; often associated with T. purpurascens, Chthamalus antennatus, and Catophragmus polymerus; Mus. Brit., Cuming, Krauss, Darwin, Stutchbury.

General Appearance.—Shell steeply conical, often rather convex; dirty or brownish white, feebly tinted with pink; external surface generally much disintegrated, and having in the upper part a pillared appearance, owing to the exposure of the upfilled, large, square, parietal tubes, and, in the lower part, a striated (and sometimes serrated) appearance, from the exposure of the parallel, approximate plates, with which the outer lamina of the shell is internally strengthened. In only a few young specimens, the whole outer lamina of the shell was well preserved; and in these the surface was very smooth, and even glossy, giving to the specimens a quite different aspect; even in partially corroded specimens, the lower part of the shell sometimes is quite smooth. Generally, the radii are developed; in most specimens they are narrow, but sometimes of moderate width; their summits are oblique, and their edges often notched or toothed. The recipient furrow, in each opposed compartment, is often almost as wide as the radius itself, and is equally notched. In some much corroded specimens there were no radii. Basal diameter of largest specimen, 1.1 of an inch.

Scuta, generally thick, sometimes very thick, with the external surface usually much corroded: articular furrow rather wide; articular ridge not very prominent; adductor ridge prominent. The rostral depressor muscle is attached in a small oblong pit, sometimes including little crests; and the lateral depressor muscle is attached to what may be described either as three or four parallel furrows or crests.

Terga, with the spur placed close to the basi-scutal angle, so that there is no basal margin on that side; spur short, with its lower end truncated and rounded; broad, even exceeding, when measured across the upper part, half the width of the valve. Articular furrow wide. Apex not beaked.

Structure of Walls and Radii.—This species differs from all the others of the genus in having only a single row (fig. 3 d) of parietal tubes; these are large, quadrangular, but elongated in the ray of the circle. They are not filled up, even at the very top of the shell, but they become thickly lined all round with compact shelly matter. When the surface of the shell is disintegrated, these upfilled tubes greatly affect, as already stated, the external appearance. The outer lamina near the basis is internally strengthened by longitudinal, sharp, approximate ridges or plates, which, also, often affect, after corrosion, the external appearance. The radii have their sutural edges formed by a set of narrow, branching ridges or septa; the ends of which, seen externally, often give a notched outline to this edge; the recipient furrows in the opposed compartments are deep, and their edges likewise are often notched: the interspaces between the branching ridges are filled up solidly. The alæ have their edges coarsely crenated. The lower edge of the sheath is not free.

The mouth and cirri present no particular characters: the third cirrus has both its rami elongated, with the terminal segment tapering. In the three posterior pairs of cirri, the tufts of little spines between the main pairs are rather large.

Affinities.—This species has no particular affinity with any other. The circumstance of there being only a single row of parietal tubes is not so important a difference as might at first be thought, inasmuch as in the other species, during their quite early youth, the walls are formed of only a single row of tubes or pores.

4. TETRACLITA PURPURASCENS. Pl. 11, fig. 1 a-1 d.

LEPAS PURPURASCENS.[110] Wood’s General Conchology, p. 55, Pl. 9, fig. 42, 1815.

BALANUS PLICATUS. Lamarck. Animaux sans Vertèbres, 1818.

------ ------ ET PUNCTURATUS. Chenu. Illust. Conch., Tab. 4, fig. 3 et 12.

CONIA DEPRESSA (!). J. E. Gray. Appendix, Dieffenbach’s Travels in New Zealand, 1843 (sine descript. vel figurâ).

Shell depressed, pale purple or dirty white, with the surface longitudinally ribbed, or corroded and granulated: radii or even sutures none, or radii well developed and broad, with their summits parallel to the basis: basis membranous: scutum transversely elongated: tergum small, with the spur extremely short and rounded.

Hab.—Sydney, New South Wales; Flinder’s Lagoon, Sir C. Hardy’s Island, Barrier Reef; King George’s Sound, Western Australia; Van Diemen’s Land; New Zealand, adhering to Pollicipes spinosus; Mus. Brit., Cuming, Stutchbury, Darwin, &c. China (?) attached to Pollicipes mitella, Mus. Brit. and Stutchbury. Generally attached to tidal rocks, sometimes to shells. Very common.

General Appearance.—Shell generally much depressed, in a few cases rather steeply conical, in one single instance cylindrical, but not much elongated. Colour, when alive, pale, but fine purple; I presume, judging from some dried specimens, sometimes dirty white. The state of the surface varies remarkably: about half the specimens (fig. 1 a) which I have seen, had the outer lamina of shell quite removed, and the surface granulated, owing to the projecting and exposed tips of the upfilled parietal tubes; the radii are not developed, and often even there is no trace of the four sutures; the rather large orifice is somewhat rounded, and the two scuta, with their surfaces disintegrated, have their middle parts deeply indenting the terga. The shell, in the other and perhaps more common condition (fig. 1 b), has the outer lamina preserved, and is longitudinally ribbed with generally at least five or six ribs on each compartment: the radii are here very wide, and extend from tip to tip of the compartments, so that their summits are parallel to the basis; they are generally covered by a brownish epidermis, thickly clothed with little spines; the orifice is neatly diamond-shaped; the apices of the opercular valves meet at a common point: these specimens are almost always smaller and younger than the granulated specimens. Altogether the specimens in the two opposite states have, in their external appearance, nothing in common, and no one, without careful examination, would ever suspect that they were specifically identical; this, however, was proved by the intermediate forms, and, in one instance, three of the compartments had their surfaces granulated, and were entirely destitute of the radii, whilst the fourth by some chance had been preserved from corrosion, was longitudinally ribbed, and had its epidermis-covered radius fully developed. The difference in the appearance of the opercular valves, in the two states, is simply owing to the degradation of their upper parts in the granulated specimens.

The basal diameter of the largest specimen was one inch, but the height only .35 of an inch.

Scuta, transversely elongated, so that the basal margin is nearly twice as long as the tergal margin: articular ridge very little prominent; articular furrow wide but shallow; adductor ridge very blunt, slightly prominent, sometimes almost absent, almost parallel to the basal margin: there are no distinct crests for the rostral or lateral depressor muscles, but some small irregular pits for the latter. In one young specimen, the lines of growth were crenated, showing a tendency in the valve to become longitudinally striated, as in the allied T. costata. In some young and immature specimens, the basal margin was deeply sinuous.

Terga, small in area, not above half that of the scuta: spur extremely short, broad, placed close to the basi-scutal angle of the valve, so that there is no basal margin on that side of the spur. The lower end and sides of the spur form one uniform curve. Articular ridge barely developed. Crests for the tergal depressores sharp and prominent.

Structure of the Shell and Radii.—The walls are very thick, and the parietal tubes small and numerous; there are sometimes from twelve to fifteen rows of tubes in the thickness of the wall. The tubes in their whole upper part are filled up solidly; and, as we have seen, are often exposed by disintegration. In very young specimens, of that size in which in T. porosa there would be only a single row of parietal tubes, there were here two or three rows. The development of the radii, as we have seen, is very capricious; the sutures even sometimes being lost. The radii, when developed, are broad, square on the summit, and covered by brownish hirsute epidermis: internally they are formed of tubes like those forming the parietes; in this respect differing from all the species except the following, T. costata. The tubes in the radii run obliquely down towards the basis; instead of in a transverse line, directly towards the opposite compartment, as might have been expected from the structure of the radii in Balanus. The alæ have their edges finely crenated. The sheath in all the specimens which I have observed is colourless; its lower edge is not free. The corium entering the parietal tubes, and lining the opercular valves, the mouth, and the anterior cirri, is generally of an extremely dark purple colour.

The Basis is entirely membranous, in which respect this species differs from all the others in the genus.

Mouth: all the trophi are unusually hairy or spinose. The labrum is deeply notched and apparently destitute of teeth on the crest. The palpi are club-shaped or enlarged at their extremities. The mandibles have the fourth tooth rudimentary.

In the Cirri, the second and third pairs are remarkably short and blunt. In one specimen the two rami of the first cirrus had respectively six and sixteen segments; those of the second, six and seven; those of the third, seven and seven; and those of the sixth cirrus twenty rather elongated segments, with a small tuft of spines between each main pair of spines.

Affinities.—This species differs from all, in its membranous basis, and in its transversely elongated scuta. It resembles T. costata, and differs from all the other species, in its radii (when developed) being square on the summit, and in being formed of tubes,—in the smallness and number of the parietal tubes,—in the absence of crests on the under side of the scuta for the rostral and lateral depressor muscles,—in the shortness and rounded form of the spur to the terga,—and, lastly, in the club-shaped palpi and small size or absence of the fourth tooth in the mandibles. T. purpurascens differs from T. costata in those points, namely, in its membranous basis and transversely elongated scuta, in which it differs from all the other species, and, moreover, in its scuta not being plainly striated longitudinally, in having more ribs on the external surface of the parietes of its shell, and in having only three pairs of main spines on the three posterior cirri.

5. TETRACLITA COSTATA. Pl. 11, fig. 2 a-2 c.

Shell depressed, whitish, generally with ten very prominent longitudinal ribs: radii broad, with their summits parallel to the basis: basis calcareous: scutum externally striated longitudinally: tergum with the spur short and rounded.

Hab.—Philippine Archipelago, Mus. Cuming. Attached to various shells, within the tidal limit.

General Appearance.—Shell whitish, probably tinged, when alive, with reddish-purple; depressed; surface perfectly preserved, smooth, but having longitudinal very prominent ribs, almost invariably ten in number; namely, three on both the rostrum and carina, and two on the two lateral compartments, with ten corresponding projections round the basal margin. Orifice passing from rounded-trigonal to diamond-shaped. The radii are very broad and square at the summit, and extend from tip to tip of the compartments. Basal diameter of largest specimen under half an inch, generally from .3 to .4 of an inch.

Scuta, of the usual sub-triangular shape, and not transversely elongated, as in T. purpurascens. External surface striated longitudinally; in many specimens there is a medial depression, or a row of very small pits, such as occur on the scuta of Balanus trigonus and lævis. The adductor ridge is moderately developed, and runs nearly parallel to the occludent margin; there are no crests for the rostral and lateral depressor muscles.

Terga: these in area equal two thirds of the scuta: the spur is short and rounded, and placed as described under T. purpurascens; but the articular ridge seems to be more prominent than in that species.

Structure of the Shell and Radii.—The parietal tubes are small, and very numerous, as in T. purpurascens. The radii are wide, square on the summits, but not so conspicuously covered by hirsute epidermis as in that species. Internally, the tubes forming the radii are smaller, and run more transversely than in T. purpurascens, that is in the normal course, as in Balanus. The edges of the alæ are nearly or quite smooth. The Basis is as distinctly calcareous, as in the other species of the genus.

Mouth: the trophi are not so hairy as in T. purpurascens; the labrum seems destitute of teeth; the palpi are club-shaped at their ends; the mandibles have only three teeth. The second and third cirri are not so short and blunt relatively to the others as in T. purpurascens. In the posterior cirri, the elongated segments carry four main pairs of spines, between which there is no intermediate tuft of fine spines.

The Affinities of this species have been fully pointed out under the last and closely related species. In external appearance, T. costata can at first hardly be distinguished from those young and pale-coloured varieties of T. purpurascens, which have their external surface not corroded, and their radii well developed.

6. TETRACLITA VITIATA. Pl. 11, fig. 3 a-3 e.

Shell white, generally tinged in the upper part with pink; surface irregular: parietal tubes very irregular: radii moderately wide, with their summits oblique: alæ with very thick crenated sutural edges: tergum with the spur not joined to the basi-scutal angle; spur with its extremity equably rounded.

Hab.—Philippine Archipelago; Barrier Reef, (Raine’s Islet), Australia; Mus. Cuming and Stutchbury. Attached to massive corals, to coral-rock, to a Tridacna, and to Tetraclita cœrulescens.

General Appearance.—Shell conical, moderately steep: white, generally with a tinge of pinkish-purple in the upper part, owing to the exposure of the tips of the upfilled parietal tubes. The lower part of the surface is generally well preserved, and is formed by very irregular, branching, longitudinal, slightly prominent, ridges. Radii of moderate width, with their summits oblique. Orifice rather large, rounded trigonal.

Scuta, rather narrow, with the upper part acuminated: the external surface generally much disintegrated, and marked with some irregular blotches of dark red. Articular ridge not prominent; articular furrow rather deep: adductor ridge, distinct from the articular ridge, pretty well developed, as are the crests for the rostral and lateral depressor muscles. In young and well preserved scuta, there is an external, medial, hyaline band, corresponding with the hollow under the adductor ridge, and caused by the thinness of the valve along this line.

Terga: externally, the carinal half is longitudinally and very feebly striated. Internally, the articular furrow is very wide, but shallow, and of unusual length, owing to the preservation of the upper part of the valve; the articular ridge is not prominent. The spur does not actually join, as in all the foregoing species, the basi-scutal angle, but is separated from it by a short piece of basal margin; its two sides are more nearly parallel than is usual, and the end is regularly rounded. It is always rather narrow, though the width varies considerably (fig. 3 d, 3 e). It extends in the same straight line with the middle of the articular furrow. The terga, though not possessing any striking characters, differ considerably in appearance from those of the other species.

Structure of the Shell and Radii.—The parietal tubes are remarkable from their irregular shapes, and unequal sizes (fig. 3 b),—hardly two resembling each other. Sometimes a single elongated tube will reach across the whole thickness of the walls. The septa between the tubes are rather thick and rugged. The tubes are generally darkly coloured from the adhering corium; they are solidly upfilled, but only in the uppermost part, by dark chocolate-red shelly matter. The radii are formed by irregularly branching ridges, with the interspaces filled up solidly. The square edges of the alæ are much thicker than in any other species, and are furnished with transverse ridges, which are sometimes slightly branched. The inner lamina of the walls near the basis, in most of the specimens, is irregularly and longitudinally ribbed, the ribs being longitudinally striated. The sheath and the upper part of the inner lamina of the parietes are clouded with chocolate-red.

The animal’s Mouth and Cirri were ill-preserved; but I was able to make out that the labrum had some strong teeth, and that the mandibles were furnished with five teeth, a greater number than in any other species. The palpi had parallel sides as usual. In the sixth cirrus, the segments had four pairs of main spines, instead of the usual number of three.

Affinities.—This species does not appear to be particularly related to any other one: perhaps it is rather nearer to the two following than to the foregoing species. The irregular parietal tubes, thick-edged alæ, form of terga, five teeth to the mandibles, and four pairs of spines to the segments of the posterior cirri, are its chief characteristics.

7. TETRACLITA CŒRULESCENS. Pl. 11, fig. 4 a-4 d.

LEPAS CŒRULESCENS. Spengler. Skrivter af Selskabet, 1 Bind., 1790.[111]

Shell with the upper part tinged greenish-blue, longitudinally ribbed: radii moderately wide, with their summits oblique: scutum with a small adductor and extremely prominent articular ridge, united together and so forming a small sub-cylindrical cavity: tergum with the spur not joined to the basi-scutal angle.

Hab.—Philippine Archipelago, attached to Balanus tintinnabulum; attached to a ship’s bottom, and to Balanus tintinnabulum, both from the Pacific Ocean; attached to a massive coral, and associated with T. vitiata, and therefore from the tropical eastern seas; Mus. Brit., Cuming, Stutchbury.

General Appearance.—Shell conical, sometimes depressed; surface with rather broad, smooth, longitudinal ridges; whitish, with the upper part greenish-blue, sometimes very feebly tinted with pink; radii white, or mottled with blueish-green, or with pink. When the outer lamina of shell has been corroded, the upfilled parietal tubes, of a dull blueish-gray colour, are exposed. The radii are moderately wide, with their summits very oblique. In basal diameter, one specimen was 1.8, and another 1.5 of an inch.

Scuta, externally furrowed very slightly in a longitudinal direction, causing the lines of growth to become a little sinuous. The valve is strong and thick; and the epidermis, when preserved, is hirsute with spines. The articular ridge is extraordinarily prominent; it projects, as measured from the external surface of the valve, to an amount equalling half the width of the valve in its widest part. The adductor ridge is very short, and is united to the bottom of the articular ridge, thus forming a small, nearly cylindrical tube, which runs up to near the apex of the valve. The inflected occludent margin is broad and coarsely toothed. The crests for both depressor muscles are not very prominent. When the scuta and terga are articulated together, owing to the great projection of the articular ridge of the scutum, its upper part is separated from the tergum (fig. 4 b), by a remarkably wide and deep, fissure-like hollow. The scutum, in some distorted specimens from a ship’s bottom, was narrow in proportion to its ordinary breadth.

Terga.—These are large. Externally, there is a broad, longitudinal depression, bounded on each side by a ridge. The carinal half is feebly striated longitudinally. A large upper portion (fig. 4 b) projects freely, and stands, when the two valves are articulated together, above the apex of the scutum, but does not form a beak as in T. porosa. Internally, the tergal margin is widely inflected; the articular furrow is very deep, but the articular ridge is not prominent. The spur is not very broad; it is separated from the basi-scutal angle by a small space of basal margin, which forms a straight line with the basal margin on the opposite side of the spur. The end of the spur is truncated, and is parallel to the basal margin.

Structure of the Shell and Radii.—The walls are thick: the parietal tubes are rather large and regular; they become solidly filled up in their upper parts. The sutural edges of the radii are formed by unusually narrow, sinuous ridges, sending off delicate denticuli on each side: the interspaces between these ridges are solidly filled up. The crenated edges of the alæ are rather thick. Neither the mouth nor the cirri offer any noticeable character; but I may observe, that, in the mandibles, the third and fourth teeth are close together; and that, in the three posterior cirri, the tufts of small spines between the main pairs are small.

Affinities.—This species is very distinct from the others, with the exception of the following T. radiata, to which it is in several respects allied. The under side of the scutum, with its great articular ridge, and the cylindrical tube formed by the union of the latter with the short adductor ridge, affords the most noticeable character.

8. TETRACLITA RADIATA. Pl. 11, fig. 5 a-5 d.

CONIA RADIATA.[112] De Blainville. Dict. Sc. Nat., 1816-1830, Pl. 164, fig. 5, 5 a, (sine descript.)

---- LYONSII. G. B. Sowerby. Genera of Recent and Fossil Shells, Plate, 1823, (sine descript.)

Shell white, with numerous approximate longitudinal ribs: radii broad, with their summits slightly oblique, internally porose: tergum with the articular ridge extraordinarily prominent, with the spur not joined to the basi-scutal angle.

Hab.—West Indies, adhering to Balanus eburneus and to Lepas anserifera. New South Wales, adhering to Tetraclita porosa. Attached to Balanus tintinnabulum, on a ship’s bottom from Sumatra; not rarely attached to Balanus tintinnabulum on ships’ bottoms; Mus. Brit., Stutchbury, and Cuming.

General Appearance.—Shell white, rather steeply conical, with numerous, approximate, rather narrow, longitudinal, rounded ribs on the walls: in a specimen half an inch in diameter, there were from eight to twelve ribs on each compartment. The outer lamina of shell seems always well preserved. Orifice rounded, trigonal. Radii white, smooth, broad, with their summits only slightly oblique. I have seen one specimen 1.2 in basal diameter, but quarter of an inch is a common size, and very young specimens are unusually frequent in collections.

Scuta broad, externally not striated longitudinally. The articular ridge is prominent, and the furrow deep, but not in so great a degree as in T. cœrulescens. The adductor ridge is only slightly prominent; it extends upwards only a little way above the lower end of the articular ridge, and does not form with the latter a cavity. There are no crests for the rostral depressor muscle, but there is a little pit, formed by the folding over of the occludent margin.

Terga.—These valves, when articulated with the scuta (fig. 5 b), project above them to an extraordinary degree, and are separated from them by a deep, fissure-like hollow, caused by the remarkable prominence of the articular ridge of the terga. The upper part of the tergum is not beaked, and does not project freely much above the sack. The valve is large; externally there is a rounded longitudinal furrow. The tergal margin is broadly inflected. The articular furrow is deep, and the articular ridge far more prominent than in any other sessile cirripede, for it projects, as measured from the outside surface, more than half the width of the valve; and consequently the valve, when viewed vertically from above, almost appears as if formed by the union of three plates, viz., the articular ridge, and the outside surface on each side of the spur. The spur is of moderate width, with the corners rounded: it is placed near, but not close to the basi-scutal angle, so that there is on this side a small portion of basal margin, forming nearly a straight line with the margin on the carinal side. In some young specimens, about the tenth of an inch in diameter, from the West Indies and from New South Wales, the spur (the position of which I found varied a little in some other specimens) was placed nearly in the middle of the valve, and very nearly at right angles to the basal margin; it is possible that these may be a distinct species, but without larger specimens to judge from, I think it more probable that this difference in the tergum is due to variation and youth.

Structure of Shell and Radii.—The parietal tubes are commonly elongated in the ray of the circle: the septa are rather thick, and strongly crenated at their basal edges. The inner lamina of the walls is strongly ribbed longitudinally. The broad radii have their sutural edges formed by ridges, with numerous and closely approximate denticuli: the interspaces between the main ridges are not soon filled up, and at the bottom, each interspace usually terminates in a pore or tube; so that the radii are not solid, as in most of the foregoing species, but porose. The alæ have their edges crenated.

Basis, calcareous, of unusual thickness; the inner, or upper surface, is striated from the centre in rays, corresponding with the ribs on the inner lamina of the walls. This striated or furrowed structure in the basis, shows a tendency to its becoming tubular or porose, as may be inferred from analogous cases in Balanus.

Animal’s body unknown.

Affinities.—This species is rather more closely allied to the last than to any other. There is a close analogy in the peculiar manner in which the scuta and terga are articulated together in the two species: in this species it is effected by the great development of the articular ridge of the tergum, and in T. cœrulescens by that of the scutum. The internally striated calcareous basis, and the internal tubular interspaces between the denticulated ridges of the radii, are peculiar characters. The white colour, the narrowly and closely ribbed parietes, and the broad radii, give this species an aspect, by which it can be easily recognised.