STRUCTURE OP TURTLES AND LIZARDS. 95 



formed by an expansion of the sternum or breast-bone, which 

 is spread out sideways, instead of being raised into a project- 

 ing keel as in Birds. The carapace and 

 plastron are covered with large horny 

 plates, variously arranged in the dif- 

 ferent species, and constituting what is 

 commonly called tortoise-shell. These 

 plates are often very beautifully disposed, 

 forming a kind of tesselated pavement ; 

 as in the common Tortoise (fig. 32), 

 which is often preserved alive in our 

 gardens. 



84. In the tribe of Lizards, the body 

 has no such covering ; but these animals, 

 having more activity than the tortoises 



/ !_ i. i n i \ -ui j Fig. 32. TORTOISE. 



(which are proverbially slow), are enabled 

 to make their escape from danger, whilst the latter are obliged 

 to trust to their bony casing for protection from it. In their 

 general form, Lizards approach Mammals, being four-footed, 

 and living for the most part on land ; but they differ from 

 them not only in their essential reptilian characters, but also 

 in several others of less consequence. Their bodies are 

 usually covered with scales, which lap over one another like 

 the tiles of a roof ; but in the Crocodile tribe, many parts of 



Fig. 33. CROCODILE. 



the surface are covered with large knotted horny plates, that 

 meet at their edges like the scales of tortoise-shell, and afford 

 an almost impenetrable covering. Although some of the 

 Lizard tribe spend a large part of their time in water, they 

 all breathe air ; but, as their respiration is very inactive, they 

 can remain for long periods beneath the surface, without 

 being obliged to come up to breathe. 



85. The tribe of Serpents may be regarded as lizards with- 

 out feet ; their spinal column is immensely prolonged ; and 



