GENERAL CHARACTERS OP REPTILES. 547 



back of the female ; and her skin, irritated by the contact of 



FIG. 325. PIPA. 



these bodies, swells and forms cells, in which the young are 

 enclosed ; the mother, thus loaded, betakes herself to the water, 

 remains there until the young have completed their meta- 

 morphoses ; and the young Pipas then quit these chambers, and 

 the mother returns to the earth. 



485. 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 ( 475) formed by the dorsal 

 vertebrae, ribs, and sternum ; the mouth destitute of teeth, but 

 furnished with a horny beak ; the skin covered with large horny 

 plates ; and the members, four in number, all formed upon the 

 same plan. 



II. SAURIA, or Lizards ; in which the dorsal vertebrae and 

 ribs are moveable, so that there is neither carapace nor plastron ; 

 the horny beak absent, but the mouth armed with teeth ; the 

 skin covered with scales; and the members usually four in 

 number. 



III. OPHIDIA, or Serpents ; which are formed upon the same 

 general plan with the Lizards, but are destitute of sternum and 

 of members. 



