Page:The Osteology of the Reptiles.pdf/153

Rh clavicles, originally pierced by the coracoid foramen, primitively forming at least the front part of the glenoid, often articulating with the sternum.

"(c) The metacoracoid of Permian reptiles, originally forming the back part of the glenoid region, lost in later reptiles (Williston), and in mammals except when preserved as a vestigial element."

It is true that such an element as the epicoracoid has not been found ossified in the early reptiles, but neither have numerous other bones in the mesenchyme of mammals, and its ossification in mammals would be nothing remarkable. A comparison of the epicoracoid of lizards (Fig. 99 ) with that of monotremes will show their identity in relations. And doubtless a similar epicoracoid filled in the interval between the coracoids above the clavicles and interclavicles in the early reptiles (Fig. 96 ). Should it eventually result that Broom's theory is the correct one, that both coracoids have remained in the Monotremata, the posterior one of which presumably represents the chief ossification of the coracoid process of higher mammals, then modern reptiles have no true coracoid, and the bone so called must be known as the procoracoid. The author believes that Gregory's theory is more probable. But, until the real homologies are fully determined, and to save confusion for the present, the terms procoracoid for the anterior bone, metacoracoid for the posterior are adopted in this work.

In all known reptiles possessing a metacoracoid, the suture separating it from the procoracoid enters the glenoid fossa (Fig. 106), except in certain therapsids (Fig. 107), where it joins the scapular suture a little in front of the articular surface. It passes directly inward to terminate in the free border. The scapula-procoracoid suture, in all the Cotylosauria and Theromorpha (Fig. 106) at least, divides nearly equally the glenoid surface in front of the metacoracoid, and is thence directed forward and upward to terminate in the front border.

The supracoracoid foramen, always present in the procoracoid (Figs. 95, 96, 99, 100, 106, 107), though not in the epicoracoid of the monotremes, and usually present in the coracoid of later reptiles (Figs. 112, 113), is absent in the Chelonia (except the Triassic Stegochelys), the Pterosauria, Ichthyosauria, Plesiosauria, Rhynchosauria (Howesia), many Phytosauria, and the Thalattosauria—chiefly