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 It may have been replaced by membrane bone.
Fig. 54. Ventral view of the shoulder-girdle and sternum of a Lizard (Loemanctus longipes) × 2. (After Parker.)
| 1. interclavicle. | 6. glenoid cavity. |
| 2. clavicle. | 7. sternum. |
| 3. scapula. | 8. xiphisternum. |
| 4. coracoid. | 9. sternal rib. |
| 5. precoracoidal process. |
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.