[Illustration: FIG. 61.–Various plastra and their horny shields. 1, _Testudo ibera_; 2, _Macroclemmys temmincki_; 3, _Cinosternum odoratum_; 4, _Sternothaerus nigricans_; 5, _Chelodina longicollis_; 6, _Chelone mydas_. _a_ or _an_, Anal shield; _abd_, abdominal shield; _f_ or _fem_, femoral; _g_ or _gul_, gular, unpaired in Fig. 3; _h_ or _hum_, humeral shield; _i_ or. _int.g_, intergular; _im_, infra-marginals; _m_, marginals; _p_ or _pect_, pectoral; _x_, in Fig. 1, inguinal shield constituting, with the axillary _xx_, the last trace of infra-marginals.] The cervical vertebrae have no ribs, except mere traces in the shape of small nodules. On the tail the ribs are often large, and, when fused with their neural supports, look like transverse processes; the whole arrangement exactly resembles that of Crocodilia. The first pair of thoracic ribs, those borne by the ninth vertebra, are peculiar. They arise from the anterior portion of the centrum, are much reduced, sometimes to mere threads of bone, and lean against the anterior rim of the second pair of ribs, in many cases without reaching the carapace. The next following ribs, those of the tenth to the sixteenth vertebra, are intimately involved in the formation of the first to seventh costal plates. The ribs of the two sacral vertebrae sometimes remain quite distinct throughout life, just touching the upper {316}ends of the iliac bones; but since these find a much more effective support in the shell, the distal ends of the sacral vertebrae fuse with the eighth, or so-called last, pair of costal plates.