204 PEOCEEDINGS OF THE GEOIOGICAL SOCIETY. [Jan. 25, 



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 Eeptiles we can only approxi- 

 mately 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. 



Prom the foregoing details it will have become apparent that the 

 most striking characteristics of the "Wealden skull are : — 1, the com- 

 pleteness 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 at- 

 tendant 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, 

 JDicynodon. In all other Eeptiles 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 skuU in Mr. Fox's collec- 

 tion, 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 obhteration 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 



* In snakes the side walls of the cranium in front of the periotic bones are 

 formed of downward extensions of the parietal and frontal bones. 



