Page:The Osteology of the Reptiles.pdf/136

118 arch. Real sacral ribs have been in all cases added from behind, since the caudal ribs have retained more or less in all animals their original attachments, while the lumbar or posterior dorsal ribs have often undergone changes. It is improbable that there has ever been any "migration" of the sacral vertebrae; that is, the first true sacral vertebrae of all animals are identical with the single sacral vertebra of Seymouria. Additional sacral ribs have been due to the gradual elongation of the basal caudal ribs and their articulation with the ilium, as shown in the tail of the alligator snapper turtle. The second and third pairs were added very early in the history of reptiles.

Remarkably, in the Lacertilia evidences of sacral ribs have not been found, the ilia being supported by transverse processes, outgrowths of the centra (Moodie).

Not only are the two sacral ribs of the Crocodilia (Fig. 121) primitive in their attachments, but the centra also have retained their primitively amphicoelous structure.

On the other hand, additional vertebrae have joined the sacrum in front, as many as three in some reptiles, but in such cases the ribs have not reverted to their primitive attachments if modified, though they may extend to the ilium. In the Ceratopsia three lumbar vertebrae have been fused with the sacrum, and their diapophyses with the ilium. Indeed in some instances (Monoclonius for instance) a vestigial free rib may remain on the first, so-called sacral vertebra. In the later pterodactyls there are several such sacro-lumbar vertebrae, and also in the Anomodontia (Fig. 119 ), groups that have been accredited with from seven to ten sacral vertebrae. In all these the ilium is greatly prolonged in front of the acetabulum. The projections from the vertebrae have often been called indiscriminately transverse processes, but that term is true only of the sacro-lumbars.

Whether or not the dinosaurs acquired the third or the fourth sacral vertebrae after their divergence from their immediate ancestral stock is perhaps a question. But two are accredited to Hallopus, a primitive type. There can be no question, however, but that the dinosaurs, both the Saurischia and the Ornithischia, descended from reptiles with but two sacral vertebrae, since the allied Crocodilia