136 TEETIAET VERTEBEATA OF THE EATtTM. 



outer wall of the deep groove, at the bottom of which the foramen lacerum anterius 

 (f.l.a.), the foramen rotundum, and the optic foramen open. This groove is continued 

 downwards and forwards almost to the angle of the pterygoids and upwards on the 

 frontals, probably terminating, as in JEleplias, at the postorbital process : in the recent 

 form the upper limb of the alisphenoid is relatively small, while the ventral or 

 pterygoid wing is greatly expanded, owing to the great size of the molar sockets, which 

 it, together with the pterygoid with which it is fused, embraces. In Palceomastodon 

 the pterygoid wing of the alisphenoid is a comparatively narrow tongue of bone closely 

 applied to and, in the adult, fused with the outer face of the pterygoid, and perhaps to 

 a small extent in front with the palatine (PI. XII. fig. 1, al.). The upper end of this 

 portion of the alisphenoid is perforated by the alisphenoid canal (al.c), the posterior 

 opening of which, as already described, lies a little below and in front of the foramen 

 ovale ; while the anterior opening is at the bottom of the deep groove above referred 

 to, and beneath the foramen lacerum anterius, with which probably the foramen 

 rotundum also opened. The boundaries between the alisphenoid and orbitosphenoid 

 cannot be determined. The optic foramen, which no doubt perforated the orbito- 

 sphenoid as usual, lies at the bottom of the groove some distance above and in firont of 

 the foramen lacerum anterius, and a shallow groove marking the course of the optic 

 nerve runs forwards from it to the orbit. The lower limit of the orbitosphenoid is 

 obscure, but it appears to have united with the upper edge of the orbital plate of the 

 palatine and perhaps also with the maxilla. 



The exact form of the pterygoid (pf.) is difficult to make out. As usual it forms 

 the posterior part of the lateral wall of the mesopterygoid fossa, embracing the 

 basisphenoid above and dying away as a ridge which is continuous posteriorly with 

 the anterior end of the tympanic ; its outer face is for the most part concealed by the 

 adherent pterygoid plate of the alisphenoid, but on the inner side the suture with 

 the palatine can be seen to run upwards and backwards from the posterior angle of the 

 vertical palatine plates to the basisphenoid. 



The posterior vertical portion of the palatine (pi.) forming the side of the meso- 

 pterygoid fossa is considerably thickened; posteriorly it joins the pterygoid in the 

 manner already described, above it riieets the vomer which extends back considerably 

 behind the level of the hard palate, and together the two bones conceal the anterior 

 part of the basisphenoid and the rest of the anterior part of the basis cranii. In front 

 the bone becomes still more thickened, so that where its horizontal plate meets its fellow 

 of the opposite side they together form not only a strong backwardly-directed process, 

 but also a large paired downwardly-projecting tuberosity, and from this the line of 

 xinion of the bones is continued forwards as a strong cristiform ridge which extends 

 throughout their whole length and on to the maxilla in front. The anterior end of 

 the palatine plates is about opposite the hinder lobe of the second molar. The suture 

 with the maxillse is transverse in front, then it runs backwards and outwards, being 



