REPTILIA. 193 



margins, leave marginal apertures towards their extremities, 

 and these openings are simply covered by a leathery skin or 

 by horny plates. 3. The margin of the carapace is com- 

 pleted by a series of bony plates, which are called the 

 "marginal plates " (fig. 548, m, m). These are variously 

 regarded as beincj dermal bones belonging to the exo- 



Fig. 548. — Transverse section of the skeleton of Chelone midas in tlie dorsal region, c, 

 Body of one of the dorsal vertebras ; n. Expanded spinous process or " neural plate " of the 

 same ; r, r, Bibs ; &, c', "Costal plates ; " m, m, Marginal plates ; p, p, Lateral elements of the 

 plastron. (After Huxley.) 



skeleton, or as being endoskeletal, and as representing the 

 ossified cartilages of the ribs (in this last case the marginal 

 plates would correspond with what are known as the "sternal 

 ribs " of Birds). Of these marginal plates the one in the 

 middle line of the carapace in front is known as the 

 " nuchal " plate, and is larger than the rest, while the 

 corresponding plate behind is termed the " pygal " plate, 

 (see fig. 550, mc and py). 



The "plastron" or ventral shield (fig. 549) is composed of 

 nine bony pieces, of which eight are in pairs, and the ninth 

 is odd. Of the paired pieces, the anterior are the cpisternals, 

 the middle pair the Jiyosterncds, and the hinder pair the 

 liyijosterncds, while the unpaired piece is termed the xii^M- 

 sternal (fig. 549, xs). The precise nature of the bones of the 

 plastron is still a matter of some doubt. Some regard them 

 as wholly corresponding with the sternum or breast-bone ; 

 others regard them as wholly integumentary ; while others, 

 again, hold — what is doubtless the correct opinion — that the 

 plastron is formed partly of bones belonging to the endo- 

 skeleton proper and representing the sternum, in part at any 

 rate, and partly of integumentary ossifications. 



VOL. II. N 



