tfHE MOST PRIMITIVE KNOWN REPTILE. 



281 



The ribs on the 4th and 5th caudals are very distinctly double- 

 headed, having a distinct emargination between the well-marked 

 tuberculum and capitulum. This feature is, I believe, known in 

 no other vertebrate. 



Seymouria bayloriensis Broili. — Outer surface of the incomplete right scapula 

 and precoracoid, X f . 



The fragment lying on the anterior end of the coracoid is clavicle. 



Reference letters; — Cl., clavicle ; F.Gl., glenoid foramen; F.Pe.Cok., precoracoid 

 foramen; F.S.Gl., supra-glenoid foramen ; Pk. Cor., precoracoid ; Sc., scapula. 



Shoulder -girdle. — The shoulder-girdle has already been well 

 described by Broili and Williston, but some features of its 

 structure have so far escaped observation and others are worthy 

 of further emphasis. The scapula is exactly as Williston has 

 described it, with a very broad supra-glenoid fossa, high up in 

 which lies the small supra-glenoid foramen. My specimen shows 

 clearly not only that it only articulates with a single coracoidal 

 element, but also that it differs from the somewhat similar con- 

 dition in Varanoops in not presenting an articular face for a 

 cartilaginous posterior coracoidal element. In fact the evidence 

 makes it quite plain that there was only a single coracoid, 

 corresponding to the anterior one of most Cotylosaurs, the 

 posterior not being represented even by a cartilage. The pre- 

 coracoid is as Williston has described it, with a very deep fossa 

 on its outer surface behind the front of the glenoid cavity. A 

 small glenoid and a much larger precoracoid foramen open into 

 the fossa. 



The glenoid cavity is represented by a deep groove, V-shaped 

 in section. This groove during life must have been filled up 

 with cartilage, its margins map out a typical screw-shaped glenoid 

 articulation. 



Sternum. — The hard matrix lying on the inner surface of the 



Text-figure 10. 



F Pk.Cor 



Pk.Cor. 



