. Water reptiles of the past and present . Fig. 14.— Rhachitomousdorsal vertebra of Eryops: n,neurocentrum or arch; pi,pleurocentrum; i, inter-centrum or hypocentrum; az,anterior zygapophysis; pz, pos-terior zygapophysis; d, diapo-physis, for tubercle of rib; p,parapophysis, for head of rib. 32 WATER REPTILES OF THE PAST AND PRESENT. Ordinarily the cervical vertebrae differ from those behind themonly in the small size or fusion of their ribs; sometimes, however,as in the Protorosauria and Pterosauria, the vertebrae may be muchelongated. The dorsal vertebrae of reptiles vary in number fromten i
. Water reptiles of the past and present . Fig. 14.— Rhachitomousdorsal vertebra of Eryops: n,neurocentrum or arch; pi,pleurocentrum; i, inter-centrum or hypocentrum; az,anterior zygapophysis; pz, pos-terior zygapophysis; d, diapo-physis, for tubercle of rib; p,parapophysis, for head of rib. 32 WATER REPTILES OF THE PAST AND PRESENT. Ordinarily the cervical vertebrae differ from those behind themonly in the small size or fusion of their ribs; sometimes, however,as in the Protorosauria and Pterosauria, the vertebrae may be muchelongated. The dorsal vertebrae of reptiles vary in number fromten in turtles and some dinosaurs to forty-three in Pleurosaurus; and under the name dorsal weinclude the so-called lumbar, asthere is seldom any real distinctionbetween the two series, save thesmaller size or the co-ossificationof the ribs of the latter. The sacrum in reptiles primi-tively consisted of a single verte-bra, which bore a large rib on eachside for the support of the early, however, a second oreven a third vertebra was addedto it from behind. The numbertwo is the rule among reptiles,both ancient and modern; amongcrawling reptiles the number neverexceeds three, but among ambulatory and flying reptiles the num-ber may be as great as in any mammal. The number of caudal vertebrae in reptiles is exceedinglyvari
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