TP:RTrARY VERTERRATA OF TJIE FAY CM. 
1 ;5G 
outer wall of tlie deep groove, at the bottom of which the foramen lacervm antervus 
{ f.l.a.), i\\o foramen rotnndum, and the o])tic foramen open. This groove is continued 
downwards and forwards almost to the angle of the pterygoids and upwards on the 
frontals, ])robably terminating, as in Elejdias, at the postorbital process; in the recent 
form the u])per 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 alis])henoid 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. I, 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 i\\e 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 probal)ly the foramen 
rotnndum also opened. The boundaries between the alisphenoid and orhitosplmioid 
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 front 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 
])alatine and perhaps also with the maxilla. 
The exact form of the [pt.) 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 sutnre 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 })terygoid in the 
manner already described, above it meets 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 
union of the bones is continued forwards as a strong enstiform ridge which extends 
tliroughout their whole length and on to the maxilla in front. The anterior end of 
the })alatine plates is about opposite the hinder lobe of the second molar. The suture 
with the maxillae is transverse in front, then it runs backwards and outwards, being 
