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- 
dine operation of their hind feet, and in part by the scull- 
ing of the long, vertically flattened tail. 
<5) 
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 ad/ the organs of the other 
