268 PHILOSOPHY OF ZOOLOGY. 



The reproductive system of reptiles exhibits few pecu- 

 liarities. The sexes are distinct, on separate individuals. 

 In some tribes, the male has external organs, while in 

 others these are wanting. Impregnation is either external 

 or internal, according to the tribes : the females are either 

 oviparous or pvo-viviparous. The young of some of the 

 genera undergo remarkable changes of form, before reach- 

 ing maturity. 



HEART WITH TWO AURICLES. 



In addition to this well marked internal character, an 

 external character may be given of more easy detection. 

 The skin is never naked, being either protected by scales, 

 knobs, or an osseous shield. 



CHELONEA. 



Body protected by a corneous shield. Body furnished 

 with feet. 



The reptiles of this group, denominated by the French 

 naturalists Cheloniens, were included in the genus Tes- 

 tudo of LINN.EUS. The body is protected dorsally and 

 ventrally, by a hollow shield, open at each end for the 

 issuing of the head and fore-feet at one time, and the tail 

 and hind-feet at another. This shield is named back-plate 

 or breast-plate^ according to its position. It is covered by 

 numerous pieces, nearly resembling horn in texture and 

 composition, exhibiting various forms and modes of union 

 with one another. In some cases, however, the external 

 covering is a continuous skin. The lateral line of junction 

 between the two plates, is more or less obviously marked 

 by the peculiar forms of the marginal plates. 



The shield is strengthened dorsally, by its intimate 

 connection with the vertebrae of the back, ventrally, by 

 the sternum, and laterally by the ribs. The vertebrae 



