The superior ridge will probably have supported the frontals, and the anterior end would terminate in the orbito-sphenoid.
The lateral ridges appear to correspond with what Prof. Huxley has described in the Ostrich as the ridge indicative of a supra-presphenoid ossification pointed out by Kölliker. The groove which is here noticed on the cerebral surface may indicate the same division. If so, the upper and anterior part of the mass would be the ethmoid.
This mass offers a considerable resemblance to the frontal portion of the skull of a dolphin (e. g. Delphinus delphis) from which the maxillary, premaxillary, palatine and nasal bones have been removed. But in the Porpoise the mesial ridge dividing the cerebral hemispheres is not prolonged so far forward as in the Pterodactyle; the cranial bones are often as smooth on the inside. Notwithstanding Wagner's assurance that the Pterodactyle skull is very like a Monitor's, he would have looked in vain for an ossification in Monitor, Iguana, or other Lizards, comparable with this mass. And although the brain is closed in front by bones in Serpents, it is by the frontal bones, which form a covering for nearly the whole of the conical cerebrum. Nor in the Crocodile is there any ossified mass in front of the brain, although the brain approximates nearer to Birds than is the case with other living Reptiles. Among Birds such a structure as that of the Pterodactyle is characteristic, but no bird has it so massive and mammal-like, though an approximation is made in some thick-skulled birds like Ciconia marabou. And in birds it usually is prolonged much further forward than appears to have been the case with Pterodactyle, where from the rapid tapering of the mass in front it appears to have ended in a vertical ridge like that in Parrots and Birds with a moveable beak. In Birds there is usually a median ridge dividing the cerebral hemispheres, but there is also often a small olfactory lobe prolonged in front of the cerebrum, to which nothing analogous is indicated in these fossils.
NATURAL MOULD OF THE BRAIN CAVITY OF A CAMBRIDGE ORNITHOSAURIAN[U]. (Cast.)
[U] For the opportunity of making this description, I am indebted to the kindness of John Francis Walker, Esq., M.A., F.G.S., F.C.S., &c., who some time since forwarded to me the whole of his rarer Ornithosaurian remains for description in the Geological Magazine,
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.