DIVISIONS OF REPTILES. 



425 



2. The ribs, which are also much flattened and exparided, and 

 constitute what are known as the "costal plates" (fig. 164, r). 

 They are generally eight in number on each side, and are 

 commonly united throughout the whole of their lateral margins 

 by sutures. In some cases, however, they 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 completed by a series of bony plates, 

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



Fig. 164.— S eleton of Tortoise (Emjis Enrofced), the plastroji being removed, ca 

 Can pace ; r Ribs, greatly expanded, and united by their edges ; s Scapular arch, 

 placed within the carapace, and carrying the fore-limbs ; / 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 tire " 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- 



