CROCODILIA. 367 



greatly prolonged, and the teeth are pretty nearly equal in size 

 and similar in form in the two jaws. 



The Procodian 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 (Eocene). 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 Crocodilia 

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

 at the present day these forms are all geographically restricted 

 in their range, and are never associated together. 



Fig. 318. Lower jaw of an Alligator. Eocene Tertiary, Isle of Wight. 



The Amphicoilian Crocodiles are characterised by their 

 biconcave vertebrae, and are entirely extinct, being confined 

 altogether to the Mesozoic period. The biconcave vertebras 

 show a decided approach to the structure of the backbone in 

 fishes ; and as the rocks in which they occur are mostly marine, 

 there can be little doubt but that these Crocodiles were, in 

 the majority of cases at any rate, inhabitants of the sea. The 

 typical members of this sub-order range from the Lias to the 

 Chalk ; and the most important genera are Teleosaurus, Steneo- 

 saurus, Dakosaurus, Makrospondylus, Goniopholis, and Sucho- 

 sanrus, the two last mentioned occurring in the fresh-water 

 deposits of the Wealden (Cretaceous). 



The Stagonolepis of the Elgin Sandstone, with its pitted dermal 

 bony scutes, is now believed to be truly referable to the Croco- 

 dilia. As before said, the Elgin Sandstones are probably Triassic. 



We may briefly consider here a group of Reptiles which 

 have been regarded as Crocodilian, but which are placed by 

 Owen in a separate order under the name of Thecodontia, and 

 which are looked upon by Huxley as being Deinosaurian. The 



