a sacrum the number between the girdles may be much greater,
Fig. 83. Sacrum and caudal vertebrae of Macrochelys (Chelonia), seen from below.
thirty-five in the mosasaurs, and as many as seventy-four in some terrestrial, legless lizards.
As has been said, there is not often the same distinction between thoracic and lumbar vertebrae that there is in mammals. There are, however, even in the Cotylosauria, examples (Fig. 164) of true lumbar vertebrae, that is, vertebrae in front of the sacrum not bearing ribs of any kind.
Sacral Vertebrae
(Fig. 83)
The sacrum of land vertebrates is composed of from one to four or five vertebrae, either fused together or separate, bearing short, stout ribs for the support of the pelvis. Rarely among the amphibians are there more than one one; certain temnospondyls and modern urodeles[1] are known to have two. It is quite certain, however, that reptiles began their career with but a single rib-bearing sacral vertebra, inasmuch as Seymouria of the Cotylosauria is known to have no more (Fig. 1). A second vertebra (Fig. 84), however, was soon added from the basal caudal by the enlargement of the ribs to come in contact with the ilium on each side. And this num-
- ↑ [Also some frogs.—Ed.]
