299 



not material ; it arises only from the greater employment 

 of the temporal muscle, and cannot be compared with the 

 differences, in the same respect, between different species 

 of the crocodile, for instance between the caiman a paupieres 

 osseuses and the gavial. 



The most striking difference, perhaps, in the whole 

 osteology of the head of crocodiles, and other lacertae, 

 is in the posterior parts of the head ; the position and mode 

 of connection of that division of the temporal bone which 

 articulates with the lower jaw, and the place of the meatus 

 auditorius. In these respects, the ichthyosaurus approaches 

 nearly to the crocodile, and recedes entirely from the other 

 lacertae. 



The vertebrae agree with neither class of saurians, but, 

 by their deeply hollowed and double concave form, approx- 

 imate to those of fishes, closely resembling those of the 

 shark in their proportions : in order further to facilitate mo- 

 tion in every direction, the annular part has a distinct arti- 

 culation with the body ; a provision peculiar to this animal. 



The humero-sternal parts consist, like those of the lacertse 

 generally, and of most oviparous animals, except fishes, of 

 the furcula or clavicular arch,* the scapulae, two bones re- 

 presenting the coracoid processes, and humeri. These last 

 support the paddle, consisting of many series of small po- 

 lygonal bones, very greatly surpassing, in number, the pha- 

 langes of quadrupeds, or even the analogous bones in the 

 fins of fish. 



Species I. /. communis. The apex of the teeth forms a 

 cone less acutely pointed than in the following 

 species. The head, figured in the Philosophical 

 Transactions for 1819, PL iii. belongs to it. 



* The crocodile is destitute, however, of the clavicular part. 



