LACERTILIA AND CROCODILIA. 415 



being American. The Gavial is distinguished by its elongated 

 snout, at the extremity of which the nostril is placed, and by 

 the fact that the teeth are pretty nearly equal in size and 

 similar in form in the two jaws. In the true Crocodiles (fig. 

 162) the fourth tooth in the lower jaw is larger than the others, 

 and forms a canine tooth, which is received into a notch ex- 

 cavated in the side of the alveolar border of the upper jaw, 

 so that it is visible externally when the mouth is closed. In 

 the Caimans or Alligators the same tooth in the lower jaw 

 forms a canine, but it is received into a pit in the palatal sur- 

 face of the upper jaw, where it is entirely concealed when the 

 mouth is shut. 



True procoelian Crocodiles occur for the first time in the 

 Greensand (Cretaceous Series) of North America. In Europe, 

 however, the earliest remains of procoelian 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. 



Sub-order 2. Amphiccclia. The Amphiccelian Crocodiles, 

 with biconcave vertebras, are entirely extinct. They have but 

 a limited geological range, extending only from the Lias to 

 the Chalk, inclusive, and being therefore strictly Mesozoic.* 

 The biconcave vertebrae show a decided approach 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 

 Crocodiles were, in the majority of cases at any rate, marine. 

 The most important genera belonging to this order are Teleo- 

 saurus, Steneosaurus, Dakosaurus, Makrospondylus, and Sucho- 

 saurus, the last being from the fresh-water deposits of the 

 Wealden (Cretaceous). 



Sub-order 3. Opisthoccelia. This sub- order, like the last, is 

 entirely extinct, and is exclusively Mesozoic, all the known 

 examples occurring in the Liassic, Oolitic, and Cretaceous 

 Rocks. The most important genera are Streptospondylus and 

 Cetiosaurus. The Cetiosaurus longus of the Upper Oolites 

 (Portland Stone) must have been the largest of all known Cro- 

 codilia, the vertebras of the tail measuring as much as seven 

 inches in length, and more than seven inches across. 



* If the so-called "Thecodont" Reptiles, such as Thecodontosaurus 

 and Belodon, belong to this sub-order, then the Amphiccelian Crocodilia 

 date from the age of the Triassic Rocks. 



