LACERTILIA AND CROCODILIA. 443 



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 



Fig. 173. — Crocodilia. Head and fore-part of t}\c body ol' the cominoii Crocodile 

 {f^rocodilus vulgaris), 



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, aiid 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 mouthl (Hence the 

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

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

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

 coi>tinued 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 tljie 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 vertebras, termed the Proccelia, AmphicxUa, and Opis- 

 thocmlia. 



Sub-order 1. Proaelia.—ln 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 Croco- 



