40 



WATER REPTILES OF THE PAST AND PRESENT 



of the head of the thigh bone, homologous with the glenoid articu- 

 lation of the pectoral girdle, which, as we have seen, was originally 

 formed by three bones, the scapula, coracoid, and metacoracoid, 

 the two latter bones, like the pubis and ischium, meeting in the 

 middle line below. In all the primitive and early reptiles the pubis 

 and ischium form a continuous plate of bone without holes in it, 

 except a small one just below the acetabulum in the pubis, called 



the obturator foramen, and correspond- 

 ing to the supracoracoid foramen Of 

 the coracoid. One may almost always 

 recognize these two bones by the presence 

 of the foramen. This "plate-like" con- 

 dition of the pelvis has been lost in all 

 late and modern reptiles by the appear- 

 ance of a larger or smaller vacuity 

 between the pubis and ischium, either 

 paired, when it corresponds quite with 

 the so-called obturator opening of mam- 

 mals, or singly in the middle. This old-fashioned character, like 

 the old-fashioned type of pectoral girdle, disappeared entirely about 

 the close of the Mesozoic period, the Choristodera, described in 

 the following pages, being the last of the kind. 



The ilium in reptiles usually has a more or less prolonged process 

 or projection turned backward by the side of the anterior caudal 

 vertebrae, but in those animals which walked erect on the hind 

 legs, the dinosaurs and pterodactyls, as also some of the more 



Fig. 22. — Pelvis of Ophiaco- 

 don: A from side; B from 

 above; pu, pubis; il, ilium; is, 

 ischium. 



