558 



MANUAL OF ZOOLOGY. 



legs simply rounded, and the feet not completely webbed. They are 

 essentially aquatic, and are voracious animals, living upon fish or Mam- 

 mals. The best-known species are the Alligator of the southern United 

 States (A. Mississippiensis), the Caiman (A . palpebrosus) of Surinam and 

 Guiana, and the "Jacare" or Spectacled Alligator (A. sclerops) of Brazil. 

 The Gavials inhabit fresh waters, and appear to be exclusively confined to 



Fig. 310. Skull of the Crocodile. 



the Ganges and other large rivers of India. The Gangetic form (Gavialis 

 Gangeticus], in spite of its numerous pointed teeth, is not so highly car- 

 nivorous as the true Crocodiles. 



True proccelian Crocodiles occur for the first time in the Greensand 

 (Cretaceous series) of North America. In Europe, however, the earliest 

 remains of proccelian Crocodiles are from the Lower Tertiary rocks (Eo- 

 cene). It is a curious fact that in the Eocene rocks of the south-west of 

 England, there occur fossil remains of all the three living types of the Croco- 

 dilia namely, the Gavials, true Crocodiles, and Alligators ; though at the 

 present day these forms are all geographically restricted in their range, and 

 are very partially associated together. 



Sub-order 2. Amphiccelia. The Amphiccelian Crocodiles, with bicon- 

 cave vertebrae, are entirely extinct. They have but a limited geological 

 range, extending only from the Trias to the Chalk inclusive, and being 

 therefore strictly Mesozoic. The biconcave vertebras show a decided ap- 

 proach to the structure of the backbone in fishes; and as the rocks in 

 which they occur are marine, there can be little doubt but that these Croco- 

 diles were, in the majority of cases at any rate, marine. The most import- 

 ant genera belonging to this order are Teleosaurus, JSelodon, Stagonokpis , 

 Steneosaurus, Dakosaurus, Makrospondylus, and Suchosaurus, the last 

 being from the fresh-water deposits of the Wealden (Cretaceous). 



Sub-order 3. Opistkoccelia. The sub-order of the Opislhoccelian Croco- 

 diles, including those forms in which the anterior trunk vertebrae are con- 

 cave behind, is one which can be only provisionally retained. Professor 

 Owen includes in this section the two genera Streptospondylus and Cetio- 

 saurus ; but the latter is referable to the Deinosauria, and will be treated 

 of when that order is considered. The genus Streptospondylus has been 

 founded on vertebrae obtained from the Oolitic and Wealden formations ; 

 but there are doubts as to the true position of the reptile to which these 

 belonged. 



