the Skull of D'lsidemodon . 305 



presumably transmitting the II., IIT., IV., V.^, and VI. 

 nerves. The posterior border of the bone unites above with 

 the periotic, but is separated from it lower down by the 

 foramen for nerves V.^ and V.^. It is noteworthy that this 

 ascending plate, which forms so large a part of the wall of 

 tlie brain-cavity, has no direct connexion with the basis 

 cranii, but is solely supported by the posterior ramus of the 

 pterygoid. One would have expected this plate to have been 

 separated by a suture as a separate epipterygoid, but there 

 is no trace of such a suture in this skull, in the type of 

 Gomphognathus polyphagus, Seeley, or in the type of Cyno- 

 gnathus crateronotus, Seeley; and if it were present one 

 would expect to see traces of it in, at any rate, the last of 

 these. 



In the Chelonia an epipterygoid is sometimes present and 

 sometimes absent, showing that it is a bone which is easily 

 functionally replaced by a process of the pterygoid. 



The anterior ramus of the pterygoid passes forward and 

 becomes widely separated from its fellow by the vomer, 

 which is suturally connected with its inner edge ; laterally 

 it expands and forms the great pterygoidal flange (fig. 3). 

 Its anterior border is formed by a sutural union with the 

 palatine, which runs transversely outwaixls until it meets the 

 short and not very closely knit suture for the transverse 

 bone ; finally, the pterygoid is produced into a powerful 

 process, which scarfs on to the dorsal surface of the palatal 

 process of the jugal. 



The pterygoid is pierced by a small foramen internally to 

 the pterygoidal flange. 



This ramus of the pterygoid passes backwards to join the 

 posteiior ramus, and forms a thin film veiling the sides of 

 the basisphenoid-vomeriue ridge; the suture separating the 

 two bones is plainly seen posteriorly and the whole length 

 of the suture is shown in one of Seeley^s Gomphognathus 

 skulls. The upper border of the bone forms a sharp ridge, 

 which continues the anterior border of the ascending plate. 

 This ridge stands up dorsally and with the central dorsal 

 ridge of the vomer forms a deep well-defined channel. 



Squamosal. — The squamosal is a very large bone which 

 has a powerful articulation with the end of the paroccipital 

 process (figs. 3 & 4) ; dorsal to this it forms a long thin 

 tongue tightly pressed to the outer and anterior face of the 

 parietal (Hg. 5] ; this i)roccss terminates in front in a point. 



