08 



ON CLASSIFICATION. 



(Fig. 33). Each ramus of the lower jaw is composed of a number 

 of pieces, and articulates with the skull, not directly, but by the 



Fig. 33. — The occipital condyle of a Crocodile's skull viewed from behind. — B.O, Basi- 

 occipital ; E.O, Ex-occipital; S.O, Supra-occipital. 



intervention of a bone — the os quadratum — with which the 

 hyoidean apparatus is not immediately connected (Fig. 34). 



Fig. 34. 



Fig. 34.- 



-The skull of a Lizard (Cyclodus). — D D, Dentary piece of the lower jaw ; 

 Qu, Os quadratum; Sq, Squamosal. 



The fore-limb of Eeptiles never takes the form of a wing, such 

 as is seen in Birds ; the " wing " of the remarkable extinct flying- 

 reptiles, the Pterodadyles, being constructed on a totally dif- 

 ferent principle from that of a bird. In no known reptile, again, 

 are the metatarsal and tarsal bones ancliylosed into one bone. 



In all Rejitilia the greater and lesser circulations are directly 

 connected together, within, or in the immediate neighbourhood 

 of, the heart ; so that the aorta, which is formed by the union 

 of two arches, contains a mixture of venous and arterial blood. 



