The ribs of reptiles are of more importance in classification than one would suppose. The primitive rib was a slender, curved bone, with the vertebral end dilated to articulate continuously with the intercentral space—that between the centra and the anterior part of the arch. And this is the condition still remaining in the tuatera. Very soon, however, the lower end of the articular surface (capitulum) became separated from the upper (tubercle) by a notch, and the ribs became distinctly double-headed. And this mode of articulation is the rule among mammals. Among later reptiles, however, there were many modifications. In nearly all the head migrated a little backward on the centrum. By the loss of the tubercle in lizards, the head became truly single-headed, and attached solely to the body; and this condition is characteristic of the order Squamata. In another large group the head of the rib gradually migrated up on the arch and on the transverse process (diapophysis), so that both head and tubercle are attached to the diapophysis; and this condition is equally characteristic of the orders of reptiles known collectively as the Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, the ribs are single-headed and attached to the end of the diapophysis. Finally in most ichthyosaurs the capitulum and tubercle both articulate with the body of the vertebra. [Illustration: FIG. 17.—_Ostodolepis_, a primitive theromorph reptile. Vertebrae from in front and side, with primitive double-headed rib and intercentrum.] Ribs primitively were probably attached to all the vertebrae to the end of the tail. In the earliest reptiles that we know they are present on all vertebrae as far back as the tenth or twelfth caudal only, those of the caudal for the most part co-ossified with the centra. The ribs of the neck vertebrae more quickly disappeared, or became fused with the vertebrae, and only in the crocodiles among living reptiles are there ribs on the atlas. The sacral ribs, on the other hand, became much larger and stouter and developed an articulation at their outer ends for the support of the ilium (Fig. 16). The so-called ventral ribs are slender ossifications in the connective tissue under the skin, on the under side of the body, and are characteristic of most reptiles. The anterior ones doubtless fused together more or less to form the sternum or breast bone, which was otherwise absent in the early reptiles. PECTORAL OR SHOULDER GIRDLE