DIVISIONS OF REPTILES. 



399 



simply covered by a leathery skin or by horny plates. 3. The 

 margin of the carapace is completed by a series of bony plates, 

 which are called the " marginal plates." These are variously 



Fig. 155. Skeleton of Tortoise (Emys Europea), the plastron being removed, ca 

 Carapace ; r Ribs, greatly expanded, and united by'their edges ; s Scapular arch, 

 placed within the carapace, and carrying the fore-limbs ; p Pelvic arch, also placed 

 within the carapace, and carrying the hind-limbs. 



regarded as being dermal bones belonging to the exoskeleton, 

 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). 



The " plastron " or ventral shield is composed of a number 

 of bony plates (nine in number), the nature of which is doubt- 

 ful. By Professor Owen, the plastron is still regarded as a 

 greatly-developed breast-bone or sternum. By Huxley and 

 Rolleston, on the other hand, the Chelonia are regarded as 

 being wholly without a sternum, and the bones of the plastron 

 are looked upon as exclusively integumentary ossifications. 

 Both the carapace and the plastron are covered by a leathery 

 skin, or more generally by a series of horny plates (fig. 156), 



