132 THE OSTEOLOGY OF THE REPTILES 



In the Chelonia it is the entoplastron (Fig. loo.) In the Croco- 

 dilia (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. loi). When present in the plesio- 

 saurs it is an oval or triangular bone, in the earlier forms imper- 

 forate, 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 temno- 

 spondyl amphibians of early Permian times, like that of the aquatic 

 reptiles, is broad and short, but that of the terrestrial types is prac- 

 tically indistinguishable from the girdle of the contemporary rep- 

 tiles. Each side, in both the amphibians and early reptiles, is com- 

 posed 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. -^ 



The three bones of the land Stegocephalia (Figs. 96 a, 108) are so 

 firmly coossified that their sutural distinctions have rarely been ob- 

 served. 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 car- 

 tilaginous 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 divi- 

 sion 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 (Hyperoda- 

 pedon) , an error. 



1 [For a different view of the fate of the two coracoids see Watson, 191 7, Journ. 

 AnaL, vol. 01; Romer, 1922, Anat. Record, vol. xxiv, pp. 39-47. — Ed.] 



