Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/330

204 cavity, while the anterior wall slants obliquely forwards. The middle fossa, which is loftier than the anterior, is wider above than below. Assuming it to have lodged the cerebellum and optic tubercles, these dimensions point to a high location of the latter, their position in the reptilian brain. The posterior fossa is the smallest, it has a laterally compressed subcylindrical shape.

It must be borne in mind that in Reptiles we can only approximately deduce the form of the brain from that of the cranial cavity, because, although in this class the brain fills the cranial cavity less incompletely than it does in fish, in which a large interspace exists between the surface of the brain and that of the cranial cavity, yet there is nothing like the coaptation of the brain and its containing chamber which occurs in the higher Vertebrata.

From the foregoing details it will have become apparent that the most striking characteristics of the Wealden skull are : — 1, the completeness of the bony brain-case ; 2, the obliteration of the sutures, particularly those of the basicranial axis ; 3, its massiveness ; 4, the great downward extension of the basisphenoid, with the attendant upward slant of the lower border of the basi-presphenoidal rod.

The completeness of the bony brain-case is an almost unique feature in a reptilian skull, being known only in one other genus, Dicynodon. In all other Reptiles the side walls and front of the cranium in advance of the periotic bones are membranous, the ali- and orbito-sphenoid, when present, being rudimentary*. But in this skull well ossified and completely developed ali- and orbito-sphenoids with a presphenoid fill this large space, which in other reptiles remains membranous. The lesser membranous space present between the supraoccipital and parietal bones in some lacertilian skulls (I am not certain whether there is not an indication of it in Dicynodon) is also absent from this.

Scarcely less remarkable in a reptilian skull is the disappearance of the sutures, particularly those in the basicranial axis, the rule being that in reptiles the primitive distinctness of the cranial elements persists throughout life. To this rule only two other exceptions are known to me : one of these is the similar skull in Mr. Fox's collection, to which I have referred in the early part of this paper ; the other is the singular ornithocephalic reptilian skull discovered by Dr. E. Bunzel, a short description of which was communicated to us last session through Prof. Huxley, who kindly afforded me an opportunity of seeing the original MS. and drawings.

In both these respects (the completeness of the bony brain-case and the disappearance of the sutures) this skull departs from the reptilian type and resembles the ornithic, which is characterized by early obliteration of the sutures and complete ossification. The curious upward slant of the part of the basi-presphenoidal rod visible at the under surface of the skull seemed at first sight to be another ornithic resemblance, the lower edge of the interorbital septum in

formed of downward extensions of the parietal and frontal bones.
 * In snakes the side walls of the cranium in front of the periotic bones are