In the Chelonia it is the entoplastron (Fig. 100.) In the Crocodilia (Fig. 121 d) and Mosasauria it is slender and free at the anterior end. The stem is short in the Ichthyosauria (Fig. 103), vestigial in the Nothosauria (Fig. 101). When present in the plesiosaurs it is an oval or triangular bone, in the earlier forms imperforate, in the later ones with a median interclavicular notch or foramen (Fig. 102). The interclavicle is absent in the Pterosauria, Dinosauria, chameleon lizards, and some plesiosaurs.
Scapular Girdle
The scapular girdle, or scapulo-coracoid of the aquatic temnospondyl amphibians of early Permian times, like that of the aquatic reptiles, is broad and short, but that of the terrestrial types is practically indistinguishable from the girdle of the contemporary reptiles. Each side, in both the amphibians and early reptiles, is composed of three bones more or less closely fused: a dorsal one, the scapula, and two ventral ones; the anterior one commonly called the procoracoid; and a posterior one, often called metacoracoid. The posterior bone was lost in all reptiles by the close of Triassic times.[1]
The three bones of the land Stegocephalia (Figs. 96 a, 108) are so firmly coössified that their sutural distinctions have rarely been observed. Among the Cotylosauria (Fig. 96 b, c) the union was less firm, or became invisible later in life; their sutural divisions have occasionally been observed. Among the Theromorpha, the posterior coracoid, the metacoracoid, is often found separated (Fig. 106), or united by a loose suture; in some forms (Fig. 96 d) it remained cartilaginous throughout life, and in all forms it probably did not ossify till growth was far advanced. Among most of the Therapsida the three bones (Fig. 107 a, b, d) fuse in maturity, but not in some, if not all, the Dinocephalia (Fig. 107 d). In the Proganosauria the division between the two bones, if present, has never been observed. In the Eunotosauria of the Upper Permian, the two bones are distinct. In no other reptile has the metacoracoid been certainly observed, though it has been affirmed in the Rhynchocephalia (Hyperodapedon), an error.
- ↑ [For a different view of the fate of the two coracoids see Watson, 1917, Journ. Anat, vol. LII; Romer, 1922, Anat. Record, vol. XXIV, pp. 39–47.—Ed.]
