TEGUMENTS OF REPTILES. 



559 



pairs of dermal bones are developed from the liyo- and hypo- 

 stcrnals, fig. 53, hs and ps: but these do not articulate with the 

 marginal series. In Freshwater and Land Tortoises the dermal 

 ossifications spread further, uniting all the parts of the plastron 



371 



372 



Outer surface, plastron, young Tortoise. 



Inner surface, plastron, young Tortoise. 



into one more or less flat floor, and all the parts of the carapace 

 into one more or less convex roof, fig. 5 1 ; side-walls being like- 

 wise now formed by the union of the hyo- and hypo-sternals 

 with the coextensive marginal plates. In all Chelonia, save the 

 TrionycidcB and Sphargis, the epiderm of the- trunk is condensed 

 into large horny scales, usually contiguous, more rarely imbricate, 

 and then only on the carajiace. They may be keeled, or rugous, 

 or scabrous, but are commonly smooth and polished, or marked 

 only by concentric lines of growth. Their growing margins 

 indent the dermal bones supporting them, forming the triradiate 

 grooves, e. g. u^ion the beginnings of the costal ■ plates in the 

 young Tortoise, fig. 370, jih, and those marked si-s5 on the 

 neural and costal plates in the Turtle, fig. 52. The large epi- 

 dermal plates of the carapace and plastron are termed ' shields ' 

 and ' tortoise-shell : ' most of them have special names in Zoology, 

 as they afford useful characters in the discrimination of genera 

 and species ; whilst the impressions they leave upon the subjacent 

 bones give similar light in the interpretation of fossil remains. 



