HEART OF REPTILES 



lizards and snakes. The size is greater than that of 

 any lizard ; but it must be remembered that a large 

 monitor lizard comes not far behind a smallish croco- 

 dile. Crocodiles are invariably aquatic, lizards are 

 very rarely so. We must necessarily mention a few in- 

 ternal characters to emphasize the separation of lizards 

 and crocodiles. In the latter the teeth are always 

 confined to the upper and lower jaws ; in lizards they 

 stray on to the bones of the palate in many cases ; 

 furthermore each tooth of a crocodile is implanted in 

 its own particular socket, while in lizards the teeth 

 are attached side by side to the bones which bear them 

 without lying at some distance apart and each like 

 a peg in a hole in these bones. The heart of the croco- 

 dile is more completely four- chambered than is that 

 of the lizard, in some of which, however, there is a 

 tendency in the same direction. On the whole the 

 lizard tribe is on a lower level of organization. Lizards 

 are excessively numerous and occur in almost all parts 

 of the world, except in very cold regions, such as the 

 extreme north. They chiefly abound in the tropics. 

 The general aspect of a lizard is familiar to every one. 

 But it is not so easy as it seems to differentiate a lizard 

 from a snake. For among the lacertilia there is a 

 distinct tendency to lose the limbs, an event which 

 has occurred in many of the families into which the 

 order is divisible, and thus produces a likeness to a 

 snake which is after all superficial. Nevertheless, the 

 two orders, the Lacertilia and the Ophidia, are very 

 nearly akin. Some differences between them will be 

 dealt with later. 



THE HELODERM LIZARD 



Of lurid coloration is the Arizona heloderm, or " gila 

 monster " as it is often termed in the United States, 

 in the southernmost of which it lives. There would 



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