194 



NATURAL HISTORY. 



therefore have their hind feet more largely webbed. In 

 the covering of both there are huge bony plates on the 

 back and tail, rising into a prominent dentated ridge on 

 the latter. This ridge is very elevated in the Crocodile 

 of the Ganges, making the tail a very efficient instrument 

 in swimming. These animals swim, in part, by the pad- 

 dling operation of their hind feet, and in part by the scull- 

 ing of the long, vertically flattened tail. 



Fig. 158. Crocodile. 



320. There is a singular arrangement of the circulation 

 in this order of reptiles. There are two ventricles in the 

 heart, as in the Mammals and the Birds ; but the red and 

 dark blood are mingled together a little distance from 

 the heart. This is not done, however, till those arteries 

 branch off which carry the blood to the anterior part of 

 the body. The result is, that the head and fore legs are 

 supplied with pure arterial blood, while all the posterior 

 parts are supplied with that mixture of red and dark 

 blood which is supplied to all the organs of the other 



