IX CARAPACE 325 



instead of ossifying, undergoes a process of cHlcitication. Ulti- 

 mately this is more or less absorbed, its place is taken by the 

 dermal bone, which forms so to speak a cast of the rib, pre- 

 serving in many cases the shape of the vanished rib, only the 

 capitular portions of which remain unaffected. The number of 

 costal plates is very constant, namely eight on each side, but 

 some fossils have nine or ten, and there are still individual 

 variations in recent forms, indicative of that number. In a 

 large Clirysemys concin7U( I find the last pair of costals clearly 

 compo.sed of at least two pairs, and this same specimen has nine 

 distinct neural plates. 



The marginal plates are originally paired, almost always 

 eleven pairs, very rarely ten or twelve ; an unpaired posterior 

 plate, the pygal, is always present, and is probably the result of 

 fusion. In the Chelonidae large fenestrae remain between the 

 costal and marginal plates, only covered by leathery unossified 

 cutis, and of course by the horny shields. In the Indian 

 fresh-water genus Batagur similar windows are gradually filled 

 up with age, and the horny shields become extremely thin and 

 almost confluent. On the other hand, in Tcstudo iwhjpliemus, 

 the bony shell, always very thin, becomes still thinner with 

 age and finally fenestrated by absorption. 



Great reduction has taken place in the carapace of the 

 Trionychidae. The American species of Trionyx have only 

 seven pairs of costal plates ; in CyclanorMs the neurals are 

 reduced to two. The whole dorsal shell is much smaller than 

 the body, and marginal plates are absent or merely vestigial. 

 It is doubtful if the ossifications in the posterior half of the 

 marginal flap of some genera are homologous wdth true marginals. 



Externally the whole shell is covered, except in the Triony- 

 chidae, in S2)hargis and CarettocJielys, with horny, epidermal shields. 

 These are phylogenetically older than the dermal plates, and 

 they do not correspond with them either in numbers or in 

 position, although there exists a general resemblance in their 

 arrangement. On the plastron- we distinguish an unpaired or 

 paired gular, and a pair of gular, humeral, pectoral, abdominal, 

 femoral, and anal shields (Fig. 66). Sometimes there are also 

 intergulars, paired in Macroclevimys and Cltchjs, unpaired in 

 Chelone ; in many of the Pleurodira an unyiaired intergulnr lies 

 behind the gulars. 



