BRITISH FOSSIL REPTILES. 95 



rostris with those of the Gangetic Gharrial, an equal degree of 

 strength and secure attachment of the teeth seem to result from 

 the two very different proportions in which the maxillary and 

 intermaxillary bones are combined together to form the upper 

 jaw. The prolongation of the snout has evidently no relation 

 to this difference ; and we are accordingly led to look for some 

 other explanation of the disproportionate' development of the 

 intermaxillary bones in the Ichthyosaurus, It appears to me 

 to give additional proof of the collective tendency of the affini- 

 ties of the Ichthyosaurus to the Lacertian type of structure. 

 Its aquatic habits necessitated the peculiar position of the nos- 

 trils, and their limited extent in that position. But in the La- 

 certians, in which they extend to the fore part of the head, their 

 interior boundary is formed by the intermaxillary bones 5 these 

 bones, therefore, conformably with the laws of organic combi- 

 nations, are extended backwards in an unusual degree, in the 

 Ichthyosaurus, to enter into their ordinary relations with the 

 nasal apertures, which are situated unusually far back in the 

 head. Before quitting the present subject I may remark that 

 in the Lacertians the median suture of the intermaxillary bones 

 is soon obliterated 3 while in the Ichthyosaur it is persistent as 

 in the Crocodile ; but this is a circumstance of minor import- 

 ance. The nasal bones in the Ichthyosaur differ from those of 

 both the Crocodilians and Lacertians in having no connexion 

 with the maxillary bones. 



In considering the conformation of the base of the cranium I 

 shall proceed in continuation to describe that part which enters 

 into the structure of the upper jaw, as these have been less ac- 

 curately described than any other part of the skull. 



The intermaxillary bones constitute a considerably greater 

 extent of the osseous palate than in either the Crocodilians or 

 Lacertians. They are not perforated as in the Crocodile, but 

 are simply emarginate on the outer side of the posterior part of 

 their palatal processes, which form, in connection with a corre- 

 sponding emargination of the palatal processes of the maxillary 

 bones, the palatal foramina. 



The maxillary bones constitute a comparatively small part of 

 the bony palate, and are, according to my observations, sepa- 

 rated widely by the intervening palatal bones and vomer, thus 

 resembling rather the Lacertian than the Crocodilian type. 



The palatal bones are joined together by a median suture, 

 except where the wedge-shaped anterior extremities of the 

 pterygoid bones are inserted into their posterior interspace, 

 the pterygoids, in like manner, being separated from each 

 other posteriorly by the intervening body of the sphenoid. The 



