REPTILES 



207 



which also approached reptiles in other more important particulars, 

 such as the possession of teeth and a long jointed tail. 



In the skeleton of the limbs the absence of clavicles may 

 be noted (see p. 145), and also the fact that both tarsus and 

 carpus are considerably modified. One of the bones of the 

 former (calcaneum) has a heel-like projection, and this is the 

 only case outside the Mammalia where anything of the kind is 

 present. 



The digestive organs (fig. 132) are interesting in a number 

 of ways. The strong conical teeth which form an irregular row 

 along the margin of each jaw are, as already mentioned, imbedded 

 in distinct sockets, and, as in 

 lizards, are replaced to an inde- 

 finite extent by successors which 

 grow up from below and push 

 them out as they get worn down 

 with wear. When the jaws are 



A. 



B. 



v.s. 



L.V. 



R.V. 



Fig. 132. Structure of Crocodile 



A, Stomach and related parts. B, Diagram of Heart and great blood-vessels: R.A. and L.A. right and 

 left auricles; R.V. and L.V. right and left ventricles cut open; v.s. ventricular septum; 1,2,4, aortic 

 arches of left side; R.AO. Right aorta; L.AO. Left aorta; SUB.CL. subclavian arteries; CAR. carotid 

 arteries; PUL, Pulmonary arteries. Arrows show course of blood. 



brought together the upper and lower teeth interlock, forming a 

 very efficient arrangement for seizing and holding. There are 

 no teeth on the roof of the mouth as in a lizard (see p. 199), 

 and the tongue is a flat rounded organ devoid of any great 

 mobility. Crocodiles are in the habit of pulling their prey under 

 water and holding it there till drowned, during which operation 

 a good deal of water must get into the mouth of the aggressor. 

 In the way already explained this water is prevented from 



