The costal plates arise, like the neurals, independently in the cutis, but they soon come into contact with the underlying cartilage of the ribs, which are long enough to reach the marginals. The ribs flatten, become surrounded by the growing membrane-bone of the plates, and the cartilage of the ribs, {325}instead of ossifying, undergoes a process of calcification. Ultimately this is more or less absorbed, its place is taken by the dermal bone, which forms so to speak a cast of the rib, preserving 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 _Chrysemys concinna_ I find the last pair of costals clearly composed 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 _Testudo polyphemus_, 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 _Cyclanorbis_ 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 with true marginals. Externally the whole shell is covered, except in the _Trionychidae_, in _Sphargis_ and _Carettochelys_ 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 _Macroclemmys_ and _Chelys_, unpaired in _Chelone_; in many of the Pleurodira an unpaired intergular lies behind the gulars.