GENERAL CHARACTERS OF REPTILES. 541 



they do not produce enough heat to have a temperature sensibly 

 higher than that of the surrounding air. The whole of their 

 body is heated or cooled at the same time as the surrounding 

 medium ; and the changes of temperature which they thus 

 experience have great influence upon all their functions. A 

 warmth of from 105 to 120 is soon fatal to most of these 

 animals; and cold tends to depress all their vital operations. 

 In winter most of them could no longer digest the food intro- 

 duced into their stomach, and do not take nourishment. Their 

 respiration also diminishes in a most remarkable manner. 



481. Like Birds, Reptiles are destitute of mammas for the 

 suckling of their young, and are reproduced by eggs ; but these 

 eggs are sometimes hatched before they come out of the body 

 (in the Viper, for example) ; and the name of ovo -viviparous 

 is given to the animals in which this phenomenon is observed. 

 The eggs of Reptiles are always of large size, and usually en- 

 closed in a shell of the consistence of parchment, in which, how- 

 ever, a small quantity of calcareous matter is sometimes contained. 



The animals generally deposit their eggs in warm sandy 

 places, where they abandon them to be hatched by atmospheric 

 warmth only ; but as the eggs are commonly retained within the 

 body of the parent until the development of the embryo has pro- 

 ceeded a considerable length, they are not exposed for any time 

 to the dangers of such a situation. Some species, however, such 

 as the Crocodiles and some Lizards, are said to watch over their 

 eggs until the exclusion of the young ; and a large Indian Ser- 

 pent, nearly allied to the Boas, known by the name of Python, 

 twines itself round its eggs ; and whilst it remains rolled round 

 its offspring, it gives out such a considerable amount of heat, that 

 the temperature of its body is sometimes raised to more than 

 104 Fahr. 



482. The class of Reptiles may be most naturally divided into 

 the four following orders, the differences between which are very 

 obvious and important. 



I. CHELONIA, or Tortoises; in which the trunk is enclosed 

 between a carapace and plastron ( 464) formed by the dorsal 

 fertebrse, ribs, and sternum, with the assistance of the dermal 



