53 2 MANUAL OF ZOOLOGY. 



by the whole of their lateral margins, leave marginal apertures 

 towards their extremities, and these openings are simply cov- 

 ered by a leathery skin or by horny plates. 3. The margin 

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



Fig. 290. Skeleton of Tortoise (Emys Europcpa), the plastron being removed. ca 

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

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

 within the carapace, and carrying the hind-limbs. 



are called the "marginal plates" (fig. 291, m m). These are 

 variously 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 the " sternal ribs " of Birds). Of 

 these marginal plates the one in the middle line of the cara- 

 pace 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. 293, nu and/^). 



The " plastron " or ventral shield (fig. 292) 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 episternals, 

 the middle pairs are the hyosternals and hyposternals^ and the 



