108 



A REVISION OF THE COTYLOSAURIA OF NORTH AMERICA 



Posterior to the axis the presacral vertebrae are very similar in form. They are 

 of the same general type found in the Diadectidct and Pareiasauridce. The centrum 

 is notochordal with wide funnels at the anterior and posterior ends; the lower line 

 is shortened for the accommodation of intercentra. The neural arches are low and 

 wide and swollen, so that the upper surface looks almost hemispherical. The spines 

 are low and short, bifurcate in the anterior vertebrae, but more posteriorly, terminat- 

 ing bluntly as if they might have been attached to some dermal plate above. The 

 zygapophyses are large and perfectly horizontal. There is no zygosphene or zygan- 

 trum. The neural arch is coossified with the centrum. The transverse processes 

 have elongate terminal faces for the single-head ribs; these are shorter than the rib 

 heads, which were either attached to the intercentra directly or by cartilage. All the 

 vertebrae were bound closely together by the wide overlapping of the zygapophyses 

 and by strong ligaments lodged in a pit at the base of each neural spine behind. The 

 transverse processes diminish in size in the posterior part of the series and the last 

 four or five presacrals did not carry ribs. 



Fig. 47. — Labidosaurus. X §. 



a, dorsal vertebrae. 1, front view; 2, top view of same vertebra. N0.4875 Am.Mus. 



b, rib of the anterior dorsal series. No. 4550 Am. Mus. 



c, shoulder girdle. CI, clavicle; 7c,interclavicle; Sep, scapula; Co, coracoid; Pe, procoracoid. After Williston. 



d, cervical and dorsal vertebra?. 



There are two sacral vertebrae which have more elevated and slender neural 

 arches than the presacrals; they are not united into a sacrum. 



The caudal vertebra have more slender neural arches, resembling in this regard 

 the sacrals rather than the presacrals. They diminish rapidly in size and are about 

 seventeen in number (Broili). 



The ribs (fig. 47, b) have expanded proximal and distal ends; the capitulum 

 and tuberculum are not separate, with the possible exception of the axial rib, but are 

 clearly indicated in the anterior dorsals. The rib of the third vertebra is bent at 

 almost a right angle, but those of the posterior vertebrae are more nearly straight. 

 The distal end is wide and spatulate. 



The anterior sacral rib is very large, with the distal end as wide as the centrum 

 is long. The rib of the second sacral is much more slender than the first and its 

 distal end underlies that of the first rib; both extend almost directly outward from 

 the vertebrae. 



