NATURAL MOULD OF THE BRAIN CAVITY OF A CAMBRIDGE ORNITHOSAURIAN[U]. (Cast.)
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NATURAL MOULD OF THE BRAIN CAVITY OF A CAMBRIDGE ORNITHOSAURIAN[U]. (Cast.)
Pl. 11.
The original specimen is in the collection of J. F. Walker, Esq., of Sidney Sussex College. When found it only displayed the front of the cerebral hemispheres, and Mr. Walker generously gave me permission to remove the investing cancellous bone and phosphate of lime, and thus exhibit the form of the cerebrum and its relations to the cerebellum. The lower part of the brain is not preserved. But adherent to the sides of the fossil are still left parts of the temporal bones, and part of the bone at the back of the orbit which closes in the brain. The form of the cerebellum is not quite perfect behind, but it must have been unusually small.
The cerebral lobes taken together are much wider from side to side than from back to front, and have a transverse elliptical outline, except for the mesial notch behind for the cerebellum. The lobes are a little flattened above, and divided from each other by a deep mesial groove, which makes each lobe convex from side to side. They are well rounded at the front and at the sides, and are a little compressed towards each other below in the region of the orbits. Behind they become covered superiorly as in birds with a greatly thickened part of the squamosal and parietal bones. The surface of the cerebrum is smooth. There is no indication of a pineal gland. The cerebellum is small, like a pea between two filberts. It is sub-hemisphercal, is placed close against the cerebrum, and appears to give off narrow lateral parts, like those seen in many birds, only that they abut against the back of the cerebral lobes as in the Hare and some Mammals. In no reptile is there a brain in which the cerebrum embraces the front of the cerebellum, or in which it attains to such an enormous size. Fœtal Mammals (e. g. the horse and the sheep), even when they have attained to a considerable bulk, and many adult mammals, still have the optic lobes dividing the cerebrum from the cerebellum as in Reptiles.
The only Mammal which shows any near approximation to this brain is the Ornithorhynchus, in which the cerebellum is very small, but the cerebrum is not so well rounded in front. The form approximates to the brain in Man. But with Birds the resemblance is so close—with the owl and the goose—that there is no character to distinguish the brain of the fossil animal from those of the recent ones. A section of the cerebrum in this specimen entirely corresponds with a section of the brain-cavity in the second skull described, as does the backward extension of the cerebrum with the extent of the cerebral cavity, and the narrow cerebellum with the narrow channel parallel to the walls of the foramen magnum, as in Gallus domesticus and Birds. The front of the brain corresponds with the cast of the front of the cerebral lobes taken from the Ethmo-sphenoid mass. Thus the specimens agree among themselves, and prove the Pterodactyle to have had a brain indistinguishable from that of a Bird. And when it is remembered how distinctive this kind of brain is, and that it approximates rather towards the higher Mammals than towards Reptiles, the fact attains unusual importance in determining the Pterodactyle's place in nature.
?NEURAL ARCH OF SACRAL VERTEBRA, ?VOMER.
Pl. 12.
Case. Comp. Tablet. Specimen.
J c 10 1—3
Frontal(?) Owen. Palæontographical, 1859 Pl. 4, fig. 6, 7, 8.
In 1859 Prof. Owen described with doubt as the Frontal of Pterodactyle, a symmetrical bone. A smaller but more perfect specimen has since been obtained for the Woodwardian Museum; and a fragment of intermediate size is in the rich collection of the Rev. T. G. Bonney. From the descriptions already given it is impossible for it to be the frontal. There is no proof that it is a skull bone. If of Pterodactyle the compressed lateral spaces could only be part of the nasal passages, or the impressions of a palatine or pterygoid articulation. And as the external surface of every specimen is keeled, and as the palatal surface of the upper jaw of every known Greensand Pterodactyle is keeled, and as the concavities slightly converge to the keel, it might be a bone from the under side of the head,—the vomer.
The smallest specimen is a compressed sub-semicircular bone 11/4 inch long, 9/16 inch high, and a 1/4 inch thick. The under surfaces converge to form a strong keel, which is flattened off behind. Above this, the posterior third of the bone is compressed obliquely to half the thickness, as though a bone had over-lapped this area on each side. If the oval spaces are nares, that bone might have been the pterygoid or palatine. Three-fifths of the remainder of the bone are taken up by the smooth oval depressions, which might be the inner walls of the nares; and above this is a margin of bone widening into the triangular compressed part in front, which, if the fossil is rightly determined, must have fitted into the posterior end of the maxillary or anterior end of the palatine bones.
A specimen collected by the Rev. T. G. Bonney is preserved on the sacral side of a left os innominatum with the keel downward. It appears to show a sutural surface from which an anterior part has come away. And if this specimen is compared with the neural arch of the sacral vertebra J.c.4.1, it will be found to correspond entirely. It is not impossible that c.10.1, 2 may be vomerine, and c.10.3 sacral, but there are no distinctive characters between the specimens to warrant such a determination.
Case. Comp. Tablet. Specimen.
J c 11 1—4
QUADRATUM. Pl. 11. and Quadrato-Jugal.
In the Woodwardian Museum are two distal ends of the quadrate bone and two other fragments showing the quadrato-jugal with it.
Quadrate. The smallest specimen is 1/2 an inch over the articular surface for the lower jaw and a quarter of an inch thick. It is concave from side to side in front where it shows a large pneumatic foramen near the basal end; it is bent from the articulation a little backward. It is convex behind; and between the foramen and the articulation sends inward and forward a great wing like that of the quadratum in birds. The specimens are broken short off and do not show any articulation above, where the bone contracts.
The distal articulation is double, like two long cones placed together; that in front having the base outward, while the hinder one has the base on the inner side. The largest specimen, which is much broken, shows the articulation half an inch thick.
Quadrato-jugal. This is a thin flat squamous bone, apparently of a transverse diamond shape, which is anchylosed to the anterior lateral margin of the quadrate, at right angles to the articulation. The lower margin is straight, as is the upper anterior margin, which appears to have received the malar bone above.
The upper posterior side is broken, but shows a large foramen near to the side of the quadrate. The base of the diamond is at the articulation, and at its apex is a small fragment of smooth surface, either part of a foramen, or the orbit of the eye.
In this specimen the articulation, which is broken, is about 3/4 of an inch wide, 3/8 of an inch thick. The remaining piece of the quadrate is an inch long. The quadrato-jugal is an inch and 3/16 high, and between its broken ends 13/4 inch long. It is thick and strong where joining the quadrate, but the rest of the bone is about an 1/8th of an inch thick.
The quadrate bone is Avian in possessing a pneumatic foramen, and Avian in the form of so much of the distal end as is preserved, and in the articulation for the lower jaw. The process which it sends inward on the inside is probably for the pterygoid bone, after the manner of Birds. Before anchylosis with the quadrato-jugal bone set in, as may be seen in J.c.11.4, the union was made by a hemispherical knob on the outside of the quadrate, as in Gallus domesticus. The squamose quadrato-jugal is a distinctive character.
Case. Comp. Tablet. Specimen.
J c 14 1—2
?PTERYGOID END OF PALATINE BONE. Pl. 12.
This determination is conjectural. Its form is such as would make it probable that it is part of the head. A more perfect specimen is seen in J.c.1.2.7.
The best specimen is a compressed sub-quadrate fragment of bone terminating at one end in a long reniform articular surface, and at the other end in a fracture where the bone is rapidly thickening. A side, regarded as the outer one, is flattened, being slightly concave in length, and slightly convex from side to side. The form of the inner side of the bone is determined by the inward curve of the thick part of the articular surface, which sends a rounded ridge obliquely on to the side, so that while it is concave from side to side at the articulation, at the fracture it is convex from side to side. All the specimens are large, the articulation being not less than an inch long.
PREMAXILLARY BONES Pl. 12.
appear to be developed as in birds. An account of their structure will be found in the notes on the species, page 112.
Case. Comp. Tablet. Specimen.
J c 12 1—6
OS ARTICULARE AND PROXIMAL END OF LOWER JAW. Pl. 12.
Prof. Owen has described in a 'Palæontographical' monograph the proximal end of a mandible in which the sutures are obliterated. But there is one specimen of a young right ramus showing the inner and under part of the mandible to be the surangular bone which unites with the angular or outer bone by a longitudinal and vertical suture traversing on the inner side the great upper groove; and on the surangular the greater part of the articular bone rests. The articulation is strong and double, consisting of a deep transverse hollow, bounded by a strong over-locking ridge in front and a slight ridge behind; and this area is divided into two tapering furrows by a strong oblique and rounded crest, which passes from behind inward and forward. Just behind the articulation is a ?pneumatic aperture, and then the upper surface tapers to the under surface, forming a heel, of which one specimen measuring an inch deep on the inside of the articulation has 3/4 of an inch still left and is more than 1/4 inch thick at the fracture. In a specimen belonging to the Rev. T. G. Bonney the outside of the jaw is 11/16 of an inch deep, and under the articulation 5/16 of an inch deep. The articular area is 3/4 of an inch wide and 6/16 of an inch long.
Seven specimens indicate four species.
The proximal end of the lower jaw is entirely Avian. The pneumatic aperture, as in birds, is placed behind the articulation, which is shaped as in many birds. Commonly in Ornithosaurians the bones are anchylosed and all trace of sutures obliterated, as in most birds. In the Goose, however, the six elements of each side are sometimes as readily separated as in reptiles. And in some Pterodactyles the bones separate.
THE DENTARY BONE Pl. 12.
The dentary bone consists of a single piece, as in birds and chelonians; and differs from both in being provided with teeth. It is described under the species O. machærorhynchus, page 113.
Case. Comp. Tablet. Specimen.
J c 17 1—39
THE TEETH. Pl. 12.
The first three teeth are usually larger than those which are placed behind them, in this respect rather resembling some fossil reptiles than Dolphins, and presenting a character like that seen in the Dimorphodon. They are placed in oblique oval sockets. They have a single fang like Cetaceans, Edentates, Reptiles, and like the premaxillary teeth of Mammals. Cambridge specimens of jaws are not sufficiently perfect to show whether the teeth are limited to the premaxillary bone; but this appears to be the case in Pterodactylus crassirostris (Goldf.), and probably in Ornithocheirus compressirostris (Owen), [Palæontographical Society, 1851, Pl. 27], and is so regarded by Professor Owen in his later writings. Yet the significance of this fact seems to have been forgotten, and Cuvier's dictum about their teeth still has influence. He says, "The teeth, by which the examination of an animal ought always to be commenced, here present nothing equivocal. They are all simple, conical, and nearly alike, as in the crocodiles, monitors, and other lizards." But, on the one hand, the Dolphins demonstrate that a mammal might have similar teeth even in the maxillary bone; and, on the other hand, since teeth in the premaxillary bone always are single-fanged, and commonly have a simple sub-conical crown, there is absolutely no evidence in the teeth of the affinities of the animal, which, so far as this portion of its economy went, might as well have been a fish or a mammal as anything else. In the succession there is nothing very distinctive. In the Crocodile one tooth comes up under another, as is commonly the case with mammals; and in mammals the fangs of the old teeth are often partially absorbed so that the teeth drop out into the mouth. In the Pterodactyle the new teeth came up on the inner side, as in the Ichthyosauria—a tribe of animals as singular in their affinities as the Ornithosauria. Occasionally specimens show a small furrow on the inner side of the fang, indicating absorption, but there is nothing to show how many times the teeth were renewed: in the Dolphins there is but one set, and in Crocodiles the teeth are replaced many times. In form and size the teeth are very variable. They are directed obliquely forward, and are curved backward and inward. They taper in an elongate cone, compressed from side to side, flattened on the outside, moderately convex on the inside; rarely the sides meet in a ridge after the plan of Pliosaurus, Megalosaurus, Dakosaurus, &c.; more frequently the lateral margins round into each other. Usually the enamel is quite smooth, sometimes, as in No. 1, it is finely striated and wrinkled. Some teeth are nearly circular and some quite straight. The ovate fang contracts below, conically, and is closed, leaving a long hollow pulp-cavity in its interior. Nos. 9, 10 show the marks of the successional teeth on their inner sides. No. 11 appears to have had the crown slightly worn at the tip during the animal's lifetime. In transverse section of the crown the tooth structure resembles Ichthyosaurus, Cetaceans, and Bats. The dentine is filled with calciferous tubes which radiate as in Ichthyosaurus, and towards the centre of the tooth are seen in transverse section to present many angles, almost like radiated corpuscles. They are separated by interspaces of their own width, and run towards the circumference, sometimes straight and sometimes wavy, parallel to each other. They send off branches usually at right angles which anastomose with the adjoining tubes. The dentine is in concentric layers, and shows layers of sub-circular cells as in the teeth of Mammals. The enamel is a thin transparent layer with fewer and finer tubes than the dentine.