CLASS REPTILIA, 



THE FIRST ORDER OF REPTILES. 



The Chelonia, 



Better known under the name of tortoises, have the 

 heart composed of two auricles and one ventricle, 

 with two unequal chambers which communicate 

 with each other. The blood of the body enters 

 into the right auricle, that of the lungs into the 

 left : but both are mingled more or less in passing 

 through the ventricle. 



These animals are distinguishable at the first 

 glance, by the double buckler in which the body is 

 enclosed, and which leaves only the head, neck, tail, 

 and four feet to pass out. 



The upper buckler, which is named carapace, or 

 back plate, is formed by the ribs, eight paks in num- 

 ber, widened and united by dentelated sutures between 

 them, and having plates adhering to the muscular 

 portions of the dorsal vertebrae, so that all these parts 

 are without motion. The lower buckler, termed 

 plastron, or breast-plate, is formed of pieces which 

 represent the sternum, and which are usually nine 

 in number. A frame-work, composed of osseous 

 pieces, in which there has been supposed to be some 

 analogy to the sternal or cartilaginous portion of the 

 ribs, surrounds the carapace with a cincture, and 

 unites all the ribs which compose it. In one sub- 



