STRUCTURE OF 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 
‘(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 
“mm 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 
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Fig. 32.—TorroIse. 
Pig. 33.—CrocopDILe. 
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 
dizard tribe spend a large part of their time in water, they 
all breathe air ; but, as their respiration is very inactive, they 
tan 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 
