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. [Illustration: 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_.] {317}[Illustration: 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.]