In most reptiles a greater or smaller number of ribs are united ventrally with a sternum; but in snakes a continuous series of similar ribs, all articulating freely with the vertebral column, extends from the third cervical vertebra to the end of the trunk. The number of ribs connected with the sternum varies from three or four in Lizards to eight or nine in Crocodiles. Those which reach the sternum are nearly always divided into vertebral, sternal, and intermediate portions, and as a rule only the vertebral portion is completely ossified. In Crocodiles a number of sternal ribs are connected with a cartilaginous arch, which is attached to the hind end of the sternum, and represents the xiphisternum. The sacral ribs connecting the vertebral column with the ilia are very distinct in Crocodiles; in these animals and _Sphenodon_ the vertebral ribs have backwardly projecting uncinate processes as in birds. In the curious arboreal lizard, _Draco volans_, the posterior ribs are long and straight, and support a parachute-like expansion of the integument used in its long flight-like leaps. In Chelonia the ribs are generally combined with the carapace. In Ichthyosauria, Sauropterygia, Crocodilia and _Sphenodon_, abdominal splint ribs occur; and probably all except the first of the paired ossifications forming the plastron of Chelonia are of similar character. Abdominal ribs have quite a different origin from true ribs, for while true ribs are cartilage bones, abdominal ribs have no cartilaginous precursors, but are simply the ossified tendons of the rectus abdominalis muscle. THE STERNUM. A sternum occurs in the following groups of reptiles: Rhynchocephalia, nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and is generally more or less rhomboidal or shield-shaped. In Pterosauria it is keeled and bears some resemblance to that of birds. It may have been replaced by membrane bone. [Illustration FIG. 54. VENTRAL VIEW OF THE SHOULDER-GIRDLE OF STERNUM OF A LIZARD (_Loemanctus longipes_) × 2. (After PARKER.) 1. interclavicle. 2. clavicle. 3. scapula. 4. coracoid. 5. precoracoidal process. 6. glenoid cavity. 7. sternum. 8. xiphisternum. 9. sternal rib.] The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia, Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia; while it is not well known in Theromorpha and Dinosauria. In the Sauropod _Brontosaurus_, however, two rounded bones occur near the base of the coracoids, and these probably represent ossified patches in a sternum, which was mainly cartilaginous; similar structures occur in _Iguanodon_. The sternum frequently remains wholly cartilaginous, especially in Lacertilia; sometimes it becomes calcified, but true ossification does not as a rule take place. APPENDICULAR SKELETON. THE PECTORAL GIRDLE.