ICHTHYOPTEBYGIA 207 



iii common with which genus the cranial structure of the 

 Ichthyosaurus exhibits a majority of lacertian characters. 



In comparing the jaws of the Ichthyosaurus tenuirostris 

 with those of the gangetic Gharrial, an equal degree of strength 

 and of alveolar border for teeth result from two very different 

 proportions in which the maxillary and premaxillary bones 

 are combined together to form the upper jaw. The prolonga- 

 tion 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 premaxillaries in the 

 Ichthijosaurus. It appears to me to give additional proof of 

 the collective tendency of the affinities of the Ichthyosaurus 

 to the lacertian type of structure. The backward or antorbital 

 position of the nostrils, like that in whales, is related to their 

 marine existence. But in the Lacertians in which the nostrils 

 extend to the fore part of the head, their anterior boundaries 

 are formed by the premaxillaries : it appears, therefore, to be 

 in conformity with the lacertian affinities of the Ichthyosaur 

 that the premaxillaries should still enter into the same relation 

 with the nostrils, although this involves an extent of anterior 

 development proportionate to the length of the jaws, the 

 forward production of which sharp-toothed instruments fitted 

 them, as in the modern dolphins, for the prehension of agile 

 fishes. 



That the Ichthyosaurs occasionally sought the shores, 

 crawled on the strand, and basked in the sunshine, may be 

 inferred from the bony structure connected with their fore fins, 

 which does not exist in any porpoise, dolphin, grampus, and 

 whale ; and for want of which, chiefly, those warm-blooded, 

 air-breathing, marine animals are so helpless when left high 

 and dry on the sands. The structure in question in the 

 Ichthyosaur is a strong osseous arch, inverted and spanning 

 across beneath the chest from one shoulder-joint to the other ; 

 and what is most remarkable in the structure of this "scapular" 



