VERTEBRATA: REPTILIA. 557 



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. 



When the exoskeleton is complete (as in Caiman), it con- 

 sists of transverse rows of quadrate bony plates disposed so as 

 to form a distinct dorsal and ventral shield, which are separ- 

 ated by soft skin in the region of the trunk, but become conflu- 

 ent in the tail. All the scutes of one row are united by suture, 

 and successive rows usually movably overlap one another. 



'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 

 immovably attached to the bottom of the mouth. (Hence the 

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

 tail is long and compressed, with two rows of keeled plates, 

 which unite about its middle to form a single crest, which is 

 continued to its extremity. The feet are palmate or semi- 

 palmate, and only the three inner toes on each foot possess 

 claws. The eyes possess three distinct lids, and there are two 

 glands under the throat secreting a musky substance. 



The Crocodilia abound in the fresh waters of hot countries, 

 and are the largest of all living Reptiles, not uncommonly 

 attaining the 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 Opistho- 

 ccelia. 



Sub-order I. Proccelia. In this sub-order are all the living members of 

 the Crocodilia, distinguished by having the bodies of the dorsal vertebrae 

 concave in front (proccelous). Three distinct types may be distinguished 

 amongst the living Crocodilia. 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. 310) the fourth tooth in the lower jaw 

 is larger than the others, and forms a canine tooth, which is received into 

 a notch excavated 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 surface of the upper jaw, where it is entirely con- 

 cealed when the mouth is shut. The Crocodiles have the hind-legs bor- 

 dered by a toothed fringe, and the toes completely united by membrane. 

 They are essentially natives of fresh water, but sometimes frequent the 

 mouths of rivers. They occur chiefly in Asia and Africa, but species are 

 found in some of the West Indian Islands. The Alligators have the hind- 



