r 4I4 MANUAL OF ZOOLOGY. 



four limbs are present, the anterior ones being pentadactylous, 

 the posterior tetradactylous. All are oviparous. 



The chief points by which the Crocodiles are distinguished 

 from their near allies, the Lacertilians, are the possession of 

 a partial bony dermal exoskeleton in addition to the ordinary 

 epidermic covering of scales, the lodgment of the teeth in 

 distinct sockets, and the fact that the mixture of venous and 

 arterial blood, which is so characteristic of Reptiles, takes 

 place, not in the heart itself, but in its immediate neighbour- 

 hood, by a communication between the pulmonary artery and 

 aorta directly after their origin. 



The only other points about the Crocodiles which require 

 special notice are that the eyes are protected by movable eye- 

 lids; the ear is covered by a movable ear-lid; the nasal cavities 

 open in front by a single nostril, and are shut off from the 

 cavity of the mouth, but open far back into the cavity of the 

 pharynx ; and lastly, the tongue is large and fleshy, and is im- 

 : movably attached to the bottom of the mouth. (Hence the 

 belief of the ancients that the Crocodile had no tongue.) 



The Crocodilia abound in the fresh waters of hot countries, 

 and are the largest of all living Reptiles, not uncommonly at- 

 taining a length of twenty feet or upwards. They are divided 

 by Owen into three sub-orders, according to the shape of the 

 dorsal vertebrae, termed the Proccelia, Amphiccelia, and Opis- 

 thoccelia. 



Sub-order i. Proccelia. In this sub-order are all the living 

 members of the Crocodilia, distinguished by having the bodies 

 of the dorsal vertebras concave in front (procoelous). The 

 best -known species of the living Crocodilia are the Gavial 

 (Gavialis Gangetica\ the Nilotic Crocodile (Crocodilus Nilot- 

 icus\ and the Alligator (Alligator Mississipemis). These three 

 forms have at the present day a very restricted geographical 



Fig. 162. Skull of young Crocodilus biporcatus (after Van der Hoven). 



range, the Gavial being exclusively confined to the rivers of Hin- 

 dostan, the, Crocodilus Niloticus being African, and the Alligators 



