PRIMITIVE REPTILES 641 



Vertebrae notochordal (or at least deeply and conically bicon- 

 cave) ; all vertebrae to the second-sixth caudal with intercentra 

 and all to the eighth or ninth with ribs, those of the lumbar, 

 sacral and basal caudal united more or less suturally with body 

 and arch, but in all cases the capitulum more or less intercentral 

 in position. Capitulum of free ribs articulating intercentrally, 

 resting more or less on the front end of the centrum, but not 

 articulating with a parapophysial process. Chevrons present 

 on all the caudals, save the first two-six and the extreme terminal 

 ones. Proatlas probably always present. Atlas composed of 

 a large free odontoid supporting the paired neural arch chiefly, 

 and an intercentrum not much larger than the one following 



Fig. 1 Proatlas, atlas and axis of Ophiacodon . 



between atlas and axis. Axis with anterior zygapophysial facets 

 for articulation with atlas. 



Interclavicle with expanded anterior part and a long posterior 

 stem; clavicles large, smooth, articulating with interclavicle, 

 scapula and sometimes with the clei thrum; scapula and coracoid 

 suturally united, fused in maturity; anterior coracoid, the so- 

 called procoracoid (the true coracoid of later reptiles) always 

 forming a part of the glenoid articulating surface; a supracoracoid 

 and a supraglenoid foramen always present; no ossified sternum. 

 Pubis and ischium plate-like, the former pierced by a pubic fora- 

 men (rarely a notch), and both meeting their mates in a median 



