122 THE OSTEOLOGY OF THE REPTH^ES 



skin bones which have sunk into the muscles. The abdominal ribs of 

 the lizards are undoubtedly true endoskeletal bones, and Fiirbringer 

 has suggested that in these animals they are new growths, supplant- 

 ing 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 paren- 

 chymatous or cartilaginous origin, and that the anterior ones fused 

 to form the sternum. ^ The so-called sternum of the modern amphib- 

 ians (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. 



Sternum 



The earliest recorded occurrence of a sternum or breastbone in rep- 

 tiles is in the Anomodontia (Fig. 94 d) 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 e) 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 cartilagi- 

 nous 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 an- 



1 [For further support of this view, see C. L. Camp, 1923, Bulletin, Amer. Mus. 

 Nat. Hist., vol. xLviii, pp. 389-393] 



