Vol. 52.] 



OF THE PLESIOSATTRIAN SKULL. 



251 



In Lariosaurus 1 the palate is essentially similar to that of 

 Plesiosaurus, but here again the pterygoids completely shut in the 

 basis cranii. The suborbital vacuity is very large, and the pterygoids 

 bear teeth, both probably primitive characters. The palate of 

 Neusticosaurus is doubtless similarly constructed, but the suborbital 

 vacuities are still larger. 



In Pistosaurus the pterygoids appear to leave the basis cranii 

 exposed for some distance, and in this respect the palate in this 

 genus is more Plesiosau- • 



rian in form than is ±ig- 2.— Diagrammatic figure of the 

 that of any other Triassic Plesiosaunan palate. 



Sauropterygian. 



Among reptiles other 

 than the Sauropterygia 

 the palate most similar to 

 that under consideration 

 is found in Sphenodon. 

 In this reptile the form 

 and the relations of the 

 bones of the palate to one 

 another and to the internal 

 nares are almost identical 

 with those above de- 

 scribed. The only differ- 

 ence of importance is that 

 the pterygoids, instead of 

 articulating directly with 

 the sides of the basisphe- 

 noid, are borne off from it 

 by downwardly - directed 

 basi-pterygoid. processes, 

 so that they come to lie at 

 a lower level than the 

 basis cranii. The conse- 

 quence of this arrangement is that the parasphenoid, here very 

 small, does not run forward between them dividing the interpterygoid 

 vacuity into two post-palatine foramina. 



Too much importance must not be attached to the similarity 

 existing between the palates of these two forms, since the Rhyncho- 

 cephalian type of palatal structure occurs in a more or less modified 

 form in many widely divergent reptilian groups, and probably there- 

 fore approaches the primitive type of structure common to the 

 ancestors of those various groups. For instance, the Ichthyosaurian 

 palate, except that the lateral wing of the pterygoid is reduced, and 

 the transverse bone consequently absent, is very like that of Sphen- 

 odon. Again, among the Anomodonts, Procolophon is, so far as 

 the palate is concerned, Rhynchocephalian ; the presence of teeth 



1 For my knowledge of the structure of the palate in this genus I am indebted 

 to Mr. Boulenger, who kindly allowed me to see a proof of his forthcoming 

 paper on the skeleton of Lariosaurus Balsami. 



