Shell composed of very many small plates arranged like mosaic. No claws .......... Sphargidae, p. [333].
The vertebrae are, sometimes in the various regions of the same individual, amphi-, opistho- or pro-coelous, or even biconvex. Traces of the chorda remain longest in the middle of the centra. Intercentra occur regularly on the first two or three cervicals, and then again in the tail as paired or unpaired nodules, or as short chevrons. The latter occasionally fuse with the caudal end of their centra. Intercentral discs of fibrous cartilage occur regularly in the neck and tail. The ribs develop originally in the same transverse level with these discs, and frequently the anterior thoracic vertebrae retain this intercentral or intervertebral position throughout life. Farther back they often show a gradual change from the intercentral to a more central and ultimately remarkable to a purely neural attachment. In all the Chelonia the ribs are devoid of the tubercular portion.
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 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.
The neural arch of the ninth vertebra rests upon its centrum; but the neural arches of the other trunk-vertebrae, although long, rest upon two centra; retaining, like the ribs, their original intercentral position; and in most cases the neuro-central sutures remain throughout life. The atlas and the last cervical vertebra deserve special attention. In many tortoises, e.g. Trionyx, Clemmys, Testudo, the three constituent parts of the atlas, namely, the neural arch, the centrum, and the intercentrum or first pair of united basiventralia, do not ankylose, but remain loosely connected; and the first centrum, instead of forming an odontoid process, remains movably attached to the second centrum, although it sometimes carries, and fuses with, the second intercentral piece. In other tortoises, e.g. Platemys and Chelys, however, all the parts of the atlas co-ossify and form a complete, solid vertebra which articulates by a concavo-convex joint with the centrum of the second vertebra. The normal number of cervical vertebrae is eight in all Chelonians. The first spinal nerve issues between occiput and atlas, all the others behind the neural arches of their vertebrae. The last, or eighth cervical, owing to the retractility of the neck, forms elaborate joints; its centre fits with a knob into a cup of the ninth, and its post-zygapophyses form broad, curved articulating concave facets for the reception of the anterior zygapophyses of the fixed ninth vertebra. In the Trionychidae the zygapophyses are most elaborate, and they alone articulate with the ninth vertebra, while the centra do not join, but remain, or rather become, separated by partial resorption. In the Chelonidae, in conformity with the non-retractile and short neck, all the cervical joints are much reduced.
Fig. 62.–1, The complete atlas of an adult Trionyx hurum. The second basiventral (white) is attached to the posterior end of the first centrum, which, not being fused with the second centrum, is not yet an odontoid process. 2, The complete atlas of an adult Trionyx gangeticus, still typically temnospondylous. 3, The first and second cervical vertebrae of an adult Platemys. 4, The complete atlas of a Chelys fimbriata.
Fig. 63.–Skull of Chelone mydas. A, from the left side; in B, the postfrontal and squamosal bones have been removed, and the broad expansions of the jugal, quadrato-jugal, parietal, and quadrate bones have been reduced in order to reduce the skull to more primitive conditions. F, Frontal; J, jugal; L.o, lateral occipital; Mx, maxillary; Op, opisthotic; Pal, palatine; Par, parietal; Prf, prefrontal; Pro, pro-otic; Pt.f, postfrontal; Ptg, pterygoid; Q, quadrate; Qj, quadrato-jugal; S.o, supra-occipital; Sq, squamosal.