As in mammals, the vertebrae of the different regions have received distinctive names, cervical, dorsal, lumbar, sacral, and caudal. The numbers of each region are far more variable than they are among mammals, the total number of vertebrae in the column varying from about thirty to more than five hundred, in certain snakes. Nor are the different regions always easily distinguishable, especially those in front of the sacrum. In the earliest reptiles there was practically no neck, and only two vertebrae, the atlas and axis, that properly can be called cervical. Very soon, however, the reptiles developed a longer neck with seven vertebrae, a number that has remained singularly constant in higher animals, especially in the mammals. In most modern reptiles there are from seven to nine; in a few lizards, five. But the number was much more inconstant among the older reptiles; some of the plesiosaurs had as many as seventy-six cervical vertebrae; some of the older lizards even had as many as eighteen. Ordinarily the cervical vertebrae differ from those behind them only in the small size or fusion of their ribs; sometimes, however, as in the Protorosauria and Pterosauria, the vertebrae may be much elongated. The dorsal vertebrae of reptiles vary in number from ten in turtles and some dinosaurs to forty-three in _Pleurosaurus_; and under the name dorsal we include the so-called lumbar, as there is seldom any real distinction between the two series, save the smaller size or the co-ossification of the ribs of the latter. [Illustration: FIG. 15.—_Ophiacodon_, a primitive theromorph reptile: proatlas, atlas, and axis, with ribs.] [Illustration: FIG. 16.—Sacrum of _Chelone_.] The sacrum in reptiles primitively consisted of a single vertebra, which bore a large rib on each side for the support of the pelvis. Very early, however, a second or even a third vertebra was added to it from behind. The number two is the rule among reptiles, both ancient and modern; among crawling reptiles the number never exceeds three, but among ambulatory and flying reptiles the number may be as great as in any mammal. The number of caudal vertebrae in reptiles is exceedingly variable, from a dozen or fifteen up to a hundred and fifty or more. In snakes but two regions are distinguishable, the caudal and precaudal, and the number altogether may reach nearly five hundred. With the exception of the first few basal caudal vertebrae (pygals) and the minute ones at the extreme tip, all caudal vertebrae of reptiles bear a slender, usually =Y=-shaped bone below in the interval between the centra, for the protection of the vessels and nerves. Because of their shape they have been called chevrons, and are really outgrowths from the intercentra.