Volume 1 — CHAPTER XXVII. (continued 2)

No fertile specimens have been found. It is probable that Callipteridium was not a true fern, and that White[1508] is correct in including it among the Pteridosperms.

Archaeopteris.

In 1852 Forbes[1509] published a brief description of some supposed fern fronds, found by the Geological Surveyors of Ireland in Upper Devonian rocks of Kilkenny, under the name Cyclopteris hibernica. The Irish specimens were more fully described by Baily[1510] in 1858. Fronds of the same type were referred by other authors to Cyclopteris, Adiantites or Noeggerathia, until Schimper[1511] proposed the generic name Palaeopteris on the ground that the fronds described by Forbes and Baily are distinguished by the nature of their fertile pinnae from the sterile leaves included in Brongniart’s provisional genus Cyclopteris. The earlier use of Palaeopteris by Geinitz for an entirely different plant led Dawson[1512] to institute the genus Archaeopteris. The genus Archaeopteris may be defined as follows:

Fronds bipinnate, reaching a considerable length (90 cm.); the stout rachis bears long linear pinnae; sterile pinnules obovate or cuneate with an entire, lobed, fimbriate, or laciniate lamina traversed by divergent dichotomously branched veins. The fertile pinnae usually occur on the lower part of the rachis; pinnules with a much reduced lamina bear numerous fusiform or oval exannulate sporangia (fig. 369, A, E, H), sessile or shortly stalked, singly, or in groups of two or three. The base of the petiole is characterised by a pair of partially adnate stipules (fig. 369, C, D), and single pinnules or scales occur in some species on the rachis between the pinnae and on the petiole.

Fig. 368. Archaeopteris hibernica. (From a specimen in the Science and Art Museum, Dublin. Rather less than ⅙ nat. size.)

Archaeopteris hibernica (Forbes). Figs. 368, 369, A–C.

The specimen from Kilkenny represented in fig. 368 has a length of over 80 cm. The upper pinnae bear numerous imbricate obovate pinnules (fig. 369, A, B) with an entire or very slightly fimbriate margin, while on the shorter lower pinnae the ultimate segments are reduced to a slender axis bearing numerous fusiform sporangia, 2–3 mm. in length. Kidston[1513] has pointed out that sporangia occasionally occur on the edge of ordinary pinnules, and he first recognised the stipular nature of the scale-like appendages which Baily noticed on the swollen petiole base (5 cm. broad) of the Irish species (fig. 369, C). Restorations of Archaeopteris hibernica have been figured by Baily[1514] and by Carruthers[1515], but the description of the fertile pinnae by the latter author requires modification in the light of Kidston’s description of the Dublin specimens.

•••••

Archaeopteris is recorded from Upper Devonian rocks of the South of Ireland, Belgium, Germany, Southern Russia, Bear Island, and Ellesmere Land in the Arctic regions, Canada, Pennsylvania, and elsewhere. Many of the specimens described under different names bear a close resemblance, which in some cases probably amounts to specific identity, to A. hibernica. A. Jacksoni originally described by Dawson[1516] and more recently by Smith and White[1517] from Devonian rocks of Maine, the Canadian type A. gaspiensis Daws., and some species figured by Lesquereux[1518] from Pennsylvania, are examples of forms which present a striking similarity in habit to the Irish species. The Belgian Devonian fossils named by Crépin[1519] Palaeopteris hibernica var. minor are regarded by him as probably identical with Goeppert’s species Cyclopteris Roemeriana from the neighbourhood of Aachen. Heer recorded Archaeopteris Roemeriana from Upper Devonian beds in Bear Island, and Nathorst[1520], who has published a more complete account of the Arctic forms, draws attention to the resemblance of some of them to A. hibernica. A species described by Schmalhausen[1521] from the Upper Devonian of Southern Russia as A. archetypus (fig. 369, D) appears to differ from A. hibernica in the slightly less reduced lamina of the fertile segments. This species has been more adequately illustrated by Nathorst[1522] from material collected in Ellesmere Land: he is unable to confirm Schmalhausen’s statement that the pinnae are spirally disposed.

The species A. fimbriata (fig. 369, G) described by Nathorst from Bear Island is characterised by the more deeply dissected lamina of the sterile pinnules. In A. fissilis Schmal. from Russia and Ellesmere Land the lamina (fig. 369, E, F) is cut up into filiform segments: a fertile pinnule of this species is represented in fig. 369, E.

Some sterile impressions figured by Krasser[1523] from Palaeozoic strata (Lower Carboniferous or Upper Devonian?) in the province of Nanshan in China as Noeggerathia acuminifissa are considered by Zeiller[1524] to be portions of an Archaeopteris or Rhacopteris frond. The resemblance to the former genus is however by no means close enough to warrant a reference to Archaeopteris. The sterile specimens described by Stur[1525] from the Culm of Altendorf as species of Archaeopteris are probably not generically identical with the Irish and Arctic species. The dichotomous branching of the rachis in A. Tschermaki and A. Dawsoni is a feature unknown in Archaeopteris. In the absence of fertile pinnae the separation of Archaeopteris from Rhacopteris is by no means easy.

Fig.

369.

Archaeopteris hibernica. Fertile pinna. Dublin Geological Survey Museum. (Reduced. After Kidston.)

A. hibernica. Pinnule. (Slightly enlarged. After Carruthers.)

A. hibernica. Base of petiole. (Dublin Museum. After Kidston.)

A. archetypus. Base of petiole: Ellesmere Land. (After Nathorst. ⅚ nat. size.)

A. fissilis. Sporangia. (Slightly enlarged. After Schmalhausen.)

A. fissilis. Sterile pinnule. Ellesmere Land. (Slightly enlarged. After Nathorst.)

A. fimbriata. Bear Island. (After Nathorst. ⅚ nat. size.)

Archaeopteris sp. Ellesmere Land. (After Nathorst. ⅚ nat. size.)

Archaeopteris was regarded by Carruthers as a fern closely allied to recent species of Hymenophyllaceae, but this conclusion was based upon an interpretation of the fertile segments which Kidston[1526] has shown to be incorrect. The latter author regarded the presence of stipules and the structure of the exannulate sporangia as evidence of a Marattiaceous alliance. In a later reference to Archaeopteris, Kidston expresses the opinion that the genus is not a true fern but a member of the Cycadofilices or Pteridosperms, a view shared by Grand’Eury[1527] and doubtless by many other palaeobotanists. The sporangia of Archaeopteris appear to be of the same type as those of Dactylotheca (fig. 290, E, p. 399). Schmalhausen gave expression to his disagreement with Nathorst and other authors who referred Archaeopteris to the Marattiaceae by proposing the distinctive group-name Archaeopterideae.

There can be little doubt that the reproductive organs of Archaeopteris so far discovered are microsporangia, and that the plant bore seeds. The sporangia are larger than those of any known fern and, as Kidston points out, they are similar to those of Crossotheca which he has shown to be microsporangia of the Pteridosperm Lyginodendron. The presence of stipules in Archaeopteris hibernica, A. fimbriata, A. archetypus (fig. 369, D) and probably throughout the genus does not materially affect the question of taxonomic position. Stipules are a characteristic feature of Marattiaceae and, in a reduced form, of Osmundaceae, but similar appendages are borne at the base of the petiole of the Cycad Ceratozamia. The occurrence of Aphlebiae on the rachis of Archaeopteris is a feature shared by the fronds of Neuropteris and other Pteridosperms.

Neuropteris.

The fronds for which Brongniart[1528] created this genus, though suspected by Stur in 1883 as wrongly classed among the ferns, have only recently been shown to be the leaves of Pteridosperms. As yet only one case is recorded in which

Neuropteris pinnae occur in organic connexion with seeds[1529], but it is almost certain that the genus as a whole must be placed in this generalised group. Renault[1530] pointed out that the petioles of Neuropteris fronds from Autun had the anatomical features of Myeloxylon (petiole of Medullosa). Since Kidston’s important discovery of seed-bearing pinnae of N. heterophylla, Grand’Eury[1531] has recorded the association of Neuropteris fronds with seeds in French Coal-fields. By some of the older authors Neuropteris was compared with Osmunda because of a similarity in venation. In the frequent dichotomy of the frond and in the occurrence of pinnules on the rachis, Neuropteris closely resembles Odontopteris[1532]: there can be little doubt as to the close relationship of the Pteridosperms possessing these two types of foliage. Neuropteris may be defined as follows:

Fronds reaching a considerable size, probably in soma cases a length of 10 metres[1533]; bi- or tri-pinnate; the rachis may be dichotomously branched (figs. 354, D; 370); both rachis and petiole bear single pinnules, those on the latter frequently differ from the normal leaflets in their larger Cyclopteroid laminae (fig. 370). Pinnules entire, rarely slightly lobed, broadly linear, attached by a small portion of the base, which is usually more or less cordate. In N. Grangeri Brongn. the pinnules are attached by a short pedicel[1534]. The midrib always dies out before reaching the blunt or pointed apex of the lamina and gives off at an acute angle numerous secondary veins characterised by their arched course and repeated forking.

Fig. 370. Neuropteris frond with Cyclopteris leaflets. English Coal-Measures. (From a block given to me by Mr Carruthers. A.C.S.)

Potonié describes the secondary veins of the pinnules of Neuropteris pseudogigantea[1535] as occasionally anastomosing, a feature which may be regarded as a step towards the reticulate venation of the closely allied genus Linopteris.

Renault[1536] described some petrified pinnules of Neuropteris in which the mesophyll shows a differentiation into upper palisade tissue and lacunar tissue below; the lower epidermis is infolded at intervals where grooves (probably stomatal) occur like those on the leaves of an Oleander (Nerium oleander).

The rachises of Neuropteris fronds are described by Grand’Eury under the generic name Aulacopteris[1537].

Neuropteris heterophylla, Brongniart[1538]. Figs. 354, E; 371.

This species is characteristic of the Lower Coal-Measures of Britain; it occurs also in the Middle Coal-Measures and is a common type in Upper Carboniferous rocks in various parts of the world. The fronds are large and tripinnate, the rachis is often dichotomously branched and Cyclopteroid pinnules may occur on the petiole. The pinnules, 5–20 mm. in length and 3–8 mm. broad, have a rounded apex (fig. 354, E, p. 535).

Fig. 371. Neuropteris heterophylla. From a specimen in the Manchester Museum. ½ nat. size. M.S.

As shown in fig. 371 which represents a primary pinna, the small pinnules on the lower branches are gradually replaced in the upper portion of the specimen by falcate segments.

Neuropteris macrophylla, Brongniart[1539]. Figs. 354, D, D′; 372.

The rachis of the large fronds of this species illustrates the dichotomous habit of many Neuropteris fronds, also the occurrence on the petiole of large Cyclopteroid pinnules (cf. fig. 370). The small piece of a pinna reproduced in fig. 372 shows the slender attachment of the segments, the blunt apex, and the Neuropteroid venation. Single pinnules of this species may be distinguished from those of N. Scheuchzeri by the blunter apex, the absence of the pair of small Cyclopteroid pinnules on the same branch and by the absence of hairs. N. macrophylla is characteristic of the Upper Coal-Measures of Britain.

Fig. 372. Neuropteris macrophylla, Brongn. From a photograph by Mr Hemingway.

Neuropteris Scheuchzeri, Hoffmann. Figs. 354, F; 373.

Fragments of this well-known Coal-Measure species were figured by Scheuchzer in his Herbarium Diluvianum[1540] as Lithosmunda minor, and by Lhywd (Luidius[1541]) as Phyllites mineralis as early as 1760. Neuropteris Scheuchzeri, so named by Hoffmann in 1826, is a type which many authors have described under different names. Lesquereux[1542] figured it as N. hirsuta from the Coal-fields of Pennsylvania, and under the same name it is recorded by Fontaine and White[1543] from Permian rocks of Virginia. The oval patches on the surface of a pinnule described by these authors as sori are certainly not of that nature. The same species is described by Bunbury[1544] from Nova Scotia as N. cordata Brongn. var. angustifolia. For a full synonymy of the species reference should be made to lists published by Kidston[1545], White[1546], and Zeiller[1547].

Fig. 373. Neuropteris Scheuchzeri. From a specimen (v. 2009) in the British Museum. ¾ nat. size.

The large tripinnate fronds are characterised by the long linear- or oval-lanceolate pinnules (fig. 373)[1548] with a pointed apex and numerous bristle-like hairs on the lamina; two much smaller Cyclopteroid segments occur at the base of the pinnae which are terminated by the linear leaflets (fig. 354, F, p. 535).

Neuropteris Scheuchzeri is characteristic of the Upper and Middle Coal-Measures of Britain and is recorded from several localities in North America and the Continent. Zalessky[1549] has recently recorded the species from the Coal-Measures of Donetz. The frequent occurrence of detached pinnules points to a caducous habit. Even single leaflets can, however, be identified by their large size, the pointed apex, and hairy lamina. The hairs are preserved as fine oblique lines simulating veins; they were so described by Roemer[1550] who took them for cross-connexions between the secondary veins and referred the pinnules to Gutbier’s genus Dictyopteris.

Another example of Neuropteris with hairy pinnules is described from the Commentry Coal-field by Renault and Zeiller as N. horrida[1551]. The oval-linear, bluntly rounded, pinnules are characterised by a median band of hairs on each surface and a narrower strip at the edge of the lamina.

Cyclopteris.

This generic name was created by Brongniart in 1828[1552] for specimens which he believed to be complete single leaves of orbicular or reniform shape similar to those of Trichomanes reniforme. The lamina is traversed by numerous dichotomously branched veins which spread from the centre of the base.

It was suspected by Lindley and Hutton[1553] that certain Cyclopteris leaves belonged to the frond of a species of Neuropteris, and some years later Lesquereux[1554] concluded that Brongniart’s genus was founded on orbicular leaflets of Neuropteris. In 1869 Roehl[1555] figured a specimen of Neuropteris bearing Cyclopteroid pinnules on its rachis. It is now universally admitted that Cyclopteris is not a distinct genus and that the specimens so named were borne as modified pinnules on the main rachis of Neuropteris and Odontopteris. It is, however, convenient to retain the name for detached leaflets which cannot be referred to the fronds on which they were borne. A specimen found by Mr Hemingway in the Upper Coal-Measures of Yorkshire and described in 1888[1556] affords a striking example of the large size attained by what was probably a frond of Neuropteris. The piece of main rachis reached a length of over 120 cm. and bore five pairs of Cyclopteris pinnules, some of which were 7 cm. long and 5 cm. broad. The complete frond must have reached a length of at least 4 metres. Fig. 370 shows some typical Cyclopteroid leaflets on the petiole of a Neuropteris frond.

Linopteris.

The Upper Palaeozoic fronds included in this genus are more familiar as species of Dictyopteris. Potonié[1557] has, however, pointed out that the creation of this name by Lamouroux in 1809 for a genus of Brown Algae which is still retained, makes it advisable to fall back upon the designation Linopteris. Gutbier[1558] proposed the genus Dictyopteris in 1835: Linopteris was first used by Presl[1559] in 1838. The fronds so named are identical with species of Neuropteris except in the anastomosis of the secondary veins; Linopteris bears to Neuropteris the same relation as Lonchopteris bears to Alethopteris. As in Neuropteris, Cyclopteroid pinnules occur on the petioles of Linopteris, but the veins form a fine reticulum. Grand’Eury[1560] records the association of Linopteris Brongniarti with seeds belonging to the genus Hexagonocarpon, a fact which points to the Pteridosperm nature of the foliage.

Some fertile pinnules of Linopteris Schutzei (Roemer) are described by Zeiller[1561] from Autun as bearing on the under surface of the lamina two rows of long and pointed sporangia, probably united in groups. The presumption is that these are microsporangia.

Fig. 374 is a reproduction of a careful drawing, originally published by Zeiller[1562], of a pinnule of the type-specimen of Gutbier’s species Linopteris neuropteroides. This species differs from Linopteris obliqua, instituted by Bunbury[1563] for specimens obtained by Lyell[1564] from the Coal-Measures of Nova Scotia, in the smaller size of the meshes. Linopteris obliqua occurs in the Upper and Middle Coal-Measures of Britain; it is recorded by Zeiller from Asia Minor, by Lesquereux[1565] from Pennsylvania, and by other authors from several European localities. The pinnules frequently occur detached from the frond and like those of some species of Neuropteris were caducous. Linopteris is rare in British strata.

Fig. 374. Linopteris neuropteroides, Gutb. (Pinnule of type-specimen. Enlarged. After Zeiller.)

Alethopteris.

The name Alethopteris, instituted by Sternberg[1566], is applied to compound fronds often reaching a considerable size, exhibiting the following features:

The linear pinnules are attached by the whole breadth of the base, with the lower edge of the lamina decurrent and usually continuous with that of the next pinnule (figs. 290, A, p. 399; 375). The ultimate segments are entire, with an acute or rounded apex and often characterised by a fairly thick lamina convex on the upper surface. From a prominent midrib, continued to the apex of the pinnule, numerous simple and forked secondary veins are given off at a wide angle, the decurrent portion of the lamina being supplied by veins direct from the axis of the pinna. In the upper part of a frond or primary pinna the pinnules may be replaced by a continuous, lobed, or entire simple lamina. The main rachis occasionally exhibits dichotomous branching, but the fronds are for the most part constructed on the pinnate plan. Single Cyclopteroid pinnules[1567] occur on the petiole of some species of the genus.

In certain species of Alethopteris the pinnules appear to have been deciduous as in Didymochlaena among recent ferns[1568]. A piece of cuticle from the upper surface of a pinnule of Alethopteris Grandini (Brongn.) figured by Zeiller[1569] shows very clearly the polygonal form and straight walls of most of the epidermal cells, those above the veins being almost rectangular. The position of the sunken stomata is revealed by small circular spaces surrounded by a circle of cells.

The absence of fertile specimens of this common genus of Upper Carboniferous plants led Stur[1570] to exclude it from the ferns. Although no seeds have so far been found in organic connexion with an Alethopteris frond, it is certain that some species, probably all, represent the foliage of Pteridosperms. Renault was the first to describe petrified specimens of Alethopteris fronds exhibiting the anatomical structure of Myeloxylon (leaf-axis of Medullosa). The calcareous nodules from English Coal-seams contain numerous fragments of the Myeloxylon type of rachis bearing Alethopteroid pinnules.

The constant association of the fronds of Alethopteris lonchitica and Trigonocarpon seeds noticed by Mr Hemingway in the Coal-Measures of Yorkshire led him to regard the species as seed-bearing: it has since been recognised as the foliage of the Pteridosperm Medullosa anglica[1571].

Grand’Eury[1572] has recorded the association in French Coal-fields of species of Alethopteris with Trigonocarpon and Pachytesta seeds.

Alethopteris lonchitica (Schlotheim)[1573]. Figs. 364, A; 290, A.

This species, described by Schlotheim in 1820 as Filicites lonchiticus and previously figured by Scheuchzer[1574], is abundant in the Middle and Lower Coal-Measures of Britain[1575]. It is characterised by large tripinnate fronds, probably quadripinnate in the lower part, bearing primary pinnae of a more or less triangular form divided into pinnate branches replaced in the apical region by linear segments. The pinnules, 8–30 mm. long and 3–5 broad, are linear- or oval-lanceolate with an obtuse apex; the upper margin of the lamina is slightly contracted at the base, while the lower edge is decurrent.

Fig. 375. Alethopteris Serlii (Brongn.). From a specimen in the York Museum. ¾ nat. size.

Alethopteris Serlii (Brongniart)[1576]. Fig. 375.

This species, figured by Parkinson in 1811, closely resembles A. lonchitica, but is distinguished by the more crowded and relatively longer pinnules which are joined to one another by a narrow connecting lamina (Fig. 375). The secondary veins in A. Serlii are rather finer and more numerous. Grand’Eury[1577] records the association of the seed Pachytesta with fronds of this species in the Coal-Measures of St Étienne.

A. Serlii is very abundant in the Upper Coal-Measures but rare in the Middle Coal-Measures of Britain[1578].

Lonchopteris.

This name was proposed by Brongniart[1579] for sterile fronds from Upper Carboniferous rocks which are practically identical with species of Alethopteris, but differ in the reticulate venation of the pinnules. It has been pointed out in a previous chapter[1580] that Lonchopteris is usually used for Palaeozoic species, the Wealden leaves, which were placed in this genus by Brongniart, being transferred to Weichselia.

There can be little doubt as to the close relationship of Lonchopteris with Alethopteris: both may be referred to the Pteridosperms. Lonchopteris rugosa Brongn.[1581] (fig. 290, B, p. 399) and L. Bricei Brongn., both British species, are fairly common in Upper Carboniferous strata. In L. rugosa, a Middle Coal-Measures species, the anastomosing secondary veins form polygonal meshes (fig. 290, B, p. 399) smaller than those of L. Bricei.

Pecopteris.

Reference has already been made to this genus in the chapter on Marattiales, so far as regards certain species of fertile fronds the sporangia of which resemble those of recent Marattiaceae. It is, however, by no means safe to assume that such Pecopteris fronds were borne on stems having the anatomical characters of ferns. The sporangia in some at least of the species may have contained microspores. In one Upper Carboniferous species usually referred to Pecopteris, P. Pluckeneti, Schlot., Grand’Eury[1582] has recorded the occurrence of seeds on the pinnules of the ordinary fronds. This species will be referred to in Volume III. The substitution of such generic names as Ptychocarpus, Asterotheca, Hawlea, Dactylotheca and others for the purely provisional designation Pecopteris indicates a step towards a conclusion as to natural affinity. The probability is that Pecopteris, as applied to Palaeozoic species, in many cases stands for the compound fronds of true ferns, but the possibility of the inclusion of those of Pteridosperms in the same category is by no means excluded. The designation Pecopteris may conveniently be retained for sterile bipinnate, tripinnate, or quadripinnate fronds bearing pinnules having the following characteristics:

Lamina short, attached to the rachis by the whole of the base and at a wide angle, with the edges parallel or slightly converging towards the usually blunt apex; adjacent pinnules may be continuous basally by a narrow lamina. A well-marked midrib extends to the apex and gives off simple or forked lateral veins almost at right angles (fig. 352, D, p. 529).

Hydathodes like those on the leaflets of Polypodium vulgare and other recent ferns[1583] are occasionally seen at the ends of the lateral veins of Pecopteris pinnules.

In addition to the examples of Palaeozoic fronds with the Pecopteris form of pinnule referred to in chapter XXII., the species Pecopteris arborescens may be briefly described.

Pecopteris arborescens (Schlotheim)[1584]. Figs. 352, D: 376.

The species named by Schlotheim Filicites arborescens in 1804 is characteristic of the Upper Coal-Measures and is recorded also from Permian strata[1585].

Fronds large; the rachis, which may reach a breadth of 3 cm.[1586], gives off long ovoid-lanceolate pinnae in two alternate rows (fig. 376); pinnules small, 1·5–4mm. long and 1–2mm. broad, contiguous, with rounded apex, attached approximately at right angles; the upper surface of the lamina is slightly convex and may be hairy[1587]. The fertile pinnules, identical in shape with the sterile, bear groups of ovoid exannulate sporangia (synangia). The midrib extends to the apex of the pinnule and gives off simple veins at a wide angle (fig. 352, D).

Our knowledge of the reproductive organs is very meagre. Grand’Eury described the synangia as consisting of 3–5 sporangia borne on a central receptacle; sporangia have been described also by Stur[1588], Renault and Zeiller[1589], and Potonié[1590], but no fertile British specimens are recorded. Stur places this species in the genus Scolecopteris, and Potonié regards the sporangia found by him on Permian fronds, which may be identical with Pecopteris arborescens, as conforming to those of the Asterotheca type. It is impossible to decide on the evidence available whether this species is a Pteridosperm or a fern, but there is a natural inclination in doubtful cases to give preference to the first of these two choices.

Fig. 376. Pecopteris arborescens (Schloth.). From the Upper Coal-Measures of Radstock. From a photograph by Dr Kidston. Reduced.

The numerous fronds from Carboniferous and Permian rocks described as species of Pecopteris exhibit a considerable range of variation in the form of the pinnules. In many species the pinnules are of the type represented in fig. 352, D; in others the lamina of the ultimate segments is slightly contracted at the base and the secondary veins are given off at a more acute angle, as in Pecopteris polymorpha, Brongn.[1591] In Pecopteris unita, Brongn., already described as Ptychocarpus unita[1592], the pinnules are joined together except in the apical region. Some fronds included in Pecopteris possess pinnules in which Pecopteroid and Sphenopteroid features are combined; P. Sterzeli, Zeill.[1593] and P. Pluckeneti, Schlot. are examples of fronds in which the pinnules are lobed as in Sphenopteris, but the base of the lamina is only slightly contracted and the venation is not that of typical Sphenopteris species.

The species to which Potonié has applied the generic name Alloiopteris[1594] also illustrates the impossibility of drawing a sharp line between Pecopteris and Sphenopteris. The fronds already described in chapter XXV. under the designation Corynepteris bear pinnules with a contracted base; in some species the lamina is lobed, but in others (fig. 354, G) it is entire with a midrib nearer one edge than the other. The species which Potonié assigns to Alloiopteris, like many other Sphenopteroid and Pecopteroid fronds, are characterised by the occurrence of an abnormal pinnule (aphlebia) at the base of each pinna (fig. 354, G, p. 535). Young fronds of Pecopteris are occasionally met with showing very clearly the circinate vernation of the pinnae as in the leaves of Cycas and Angiopteris represented in fig. 220, p. 283. The genus Spiropteris was created by Schimper[1595] for coiled unexpanded fronds of fossil ferns; it is however superfluous to apply a distinctive term to specimens of this kind.

The designation Pecopteris is employed chiefly for leaves of Palaeozoic age which are unknown in the fertile state, or do not afford sufficient evidence as to the nature of the sporangia to justify the substitution of a special generic name. Many Mesozoic species have also been referred to Pecopteris, but most of these are more appropriately included in Brongniart’s later genus Cladophlebis. The pinnules of Cladophlebis, as Brongniart pointed out, are intermediate between Pecopteris and Neuropteris; they are usually attached by the whole breadth of the base, as in Pecopteris, but the more acute origin, more arched form, and more frequent dichotomy of the lateral veins are features shared by Neuropteris. As a rule, Mesozoic sterile fronds with straight or folded, entire or dentate pinnules are of the Cladophlebis type: this genus is especially characteristic of Rhaetic and Jurassic floras. Examples of Cladophlebis pinnules are shown in figs. 256, 257 (pp. 340, 342). It is to be regretted that authors do not make more use of the generic name Cladophlebis in describing sterile fronds, instead of following the misleading and unscientific practice of employing such genera as Pteris, Asplenites, and others on wholly insufficient grounds.