134 



THE OSTEOLOGY OF THE REPTILES 



which has functionally disappeared in all modern reptiles, and not a 

 fusion of the two, now seems complete. The coracoid of Hzards, 

 crocodiles, and Sphenodon is homologous with the anterior of the 

 two bones, the so-called procoracoid. It was Howes and Lydekker 

 who first reached this conclusion, and who proposed the name meta- 

 coracoid for the posterior bone. Whether this conclusion is the right 

 one so far as the monotreme mammals are concerned is still a de- 



FiG. 107. Pectoral girdles (Therapsida): A, Ga/^o/)J (Dromasauria). Natural size. B, 

 Gtf/^fA/r«i (Dromasauria). Natural size. C, G«/^/)«j (Dromasauria). About three fourths 

 natural size. D, Moschops (Dinocephalia). One fifth natural size. 



batable question. The two coracoids in these mammals seem, and 

 generally are considered to be, homologous with those of the early 

 reptiles. Broom has suggested that in the evolution of the mammals 

 the posterior bone, that is, the metacoracoid, was retained, though 

 lost in the reptiles. Gregory, however, has offered another solution 

 of the problem that would homologize the anterior or "procoracoid" 

 of the reptiles with the posterior bone of the Monotremata. He 

 thinks that three elements are involved in the problem of their evo- 

 lution : 



"(a) The epicoracoid of Sphenodon, lizards and monotremes, a 

 sheet of bone lying immediately above the clavicles, and never reach- 

 ing the glenoid surface. 



" ib) The true coracoid, or so-called procoracoid, lying behind the 



