Fig. 14.—Rhachitomous dorsal vertebra of Eryops: n, neurocentrum or arch; pl, pleurocentrum; i, intercentrum or hypocentrum; az, anterior zygapophysis; pz, posterior zygapophysis; d, diapophysis, for tubercle of rib; p, parapophysis, for head of rib.
In front of the atlas, that is, between it and the skull, there was, in all early reptiles, as well as in some later ones, like the crocodiles and tuatera, the remnant of what is believed to have been another vertebra, of which only the arch remains, and which is called the proatlas. In its earliest condition it articulated with the skull in front and the arch of the atlas behind.
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.
Fig. 15.—Ophiacodon,
a primitive theromorph
reptile: proatlas, atlas,
and axis, with ribs.
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.