Often the vertebrae are short throughout the vertebral column; sometimes the posterior cervicals and the dorsals are elongated and very robust. The trunk or body proper was never much elongated in the plesiosaurs, having only from twenty-five to thirty vertebrae. The tail was always shorter than the trunk, and it tapered rapidly to the extremity; in some specimens it has been observed to turn up slightly near the extremity, as though for the support of a small terminal fin. [Illustration: FIG. 34.—Cervical vertebrae, from the side and behind, and dorsal vertebra from in front of _Polycotylus_, a Cretaceous plesiosaur: _az_, anterior zygapophysis; _pz_, posterior zygapophysis, _r_, _r_, _r_, cervical ribs; _d_, articulation of dorsal rib.] The ribs in the cervical region are short, but so locked together posteriorly as not to permit much lateral motion. They are sometimes double-headed in the neck, sometimes single-headed, but both heads when present articulate or are attached to the body of the vertebrae, distinguishing them at once from those of other animals, except the ichthyosaurs. In the dorsal region the ribs are attached high on the arch to the extremity of the stout transverse processes by a single head, very much as they are in some cetaceans, and quite unlike the condition in any other known reptile. They end freely below, having no attachment to a breast bone or other bony parts. Because of their shape and position as frequently found, the body in life must have been flattened from above downward, and broad; indeed, this shape is quite certain because of the very broad expanse of the coracoids, between the articulations of the front legs. [Illustration: FIG. 35.—Pectoral girdle of _Trinacromerum_ from above: _ic_, interclavicle; _cl_, clavicle; _sc_, scapula; _c_, coracoid.]