Page:The Osteology of the Reptiles.pdf/152

134 which has functionally disappeared in all modern reptiles, and not a fusion of the two, now seems complete. The coracoid of lizards, 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 metacoracoid for the posterior bone. Whether this conclusion is the right one so far as the monotreme mammals are concerned is still a debatable 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.



. 107. Pectoral girdles (Therapsida): A, Galeops (Dromasauria). Natural size. B, Galechirus (Dromasauria). Natural size. C, Galepus (Dromasauria). About three fourths natural size. D, Moschops (Dinocephalia). One fifth natural size.

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 evolution:

"(a) The epicoracoid of Sphenodon, lizards and monotremes, a sheet of bone lying immediately above the clavicles, and never reaching the glenoid surface.

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