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122 skin bones which have sunk into the muscles. The abdominal ribs of the lizards are undoubtedly true endoskeletal bones, and Fürbringer has suggested that in these animals they are new growths, supplanting the dermal parasternals which have long since disappeared, and that they represent the ends of the dorsal ribs, or outgrowths from them.

That they and the sternum to which they are supposed to have given origin are really the ends of true ribs is improbable, since no other tetrapods are known in which the dorsal ribs meet on the under side of the body, or even approach each other. It would seem more reasonable that the abdominal ribs of all reptiles are of parenchymatous or cartilaginous origin, and that the anterior ones fused to form the sternum. The so-called sternum of the modern amphibians (there was no sternum of any kind in the Stegocephalia) is an ossification of the myocomata, not derived from the dorsal ribs, and is thought not to be homologous with the sternum of reptiles.

The earliest recorded occurrence of a sternum or breastbone in reptiles is in the Anomodontia (Fig. 94 ) where, according to Broom, it is generally present and ossified. It is figured in Keirognathus as a small, subquadrilateral bone lying over the posterior extremities of the coracoids and distal end of the interclavicle. Only rarely does it occur as an ossification in other reptiles, the best examples of which are the Pterosauria (Fig. 94 ) where, as a broad, shallow concave bone, it covers the whole under side of the thoracic region with a stout manubrium-like process in front, but without a true keel. On either side of the base of the median anterior protuberance it gives articulation to the elongate coracoid. Its lateral margins have articular facets for four or five, sometimes ossified, sternal ribs. Posteriorly in the middle it is contiguous with the parasternal ribs.

In many reptiles the sternum is wholly absent, even as a cartilaginous element. There was no space, even for a rudimentary one, in the Ichthyosauria and Sauropterygia back of the united coracoids and in front of the parasternals. It has been thought that its absence in these orders is due to its loss; it is more probable that their