78 



A REVISION OF THE COTYLOSAURIA OF NORTH AMERICA 



fourth vertebra. The capitulum and tuberculum are distinct, the first reaching to 

 the intercentrum and the latter touching the end of the transverse process. The 

 fourth resembles the third, but is much larger. The capitulum and tuberculum are 

 separated by a slight notch only. The capitulum is attached to the facet on the 

 anterior edge of the fourth vertebra. The whole rib is larger and the point on the 

 posterior edge larger than the third. 



The fifth rib is much larger than the third and fourth, but the posterior prolonga- 

 tion is less marked than on the fourth. The capitulum and tuberculum are hardly 

 distinct, thus corresponding to the position of the facets on the vertebra; the one on 

 the anterior edge of the centrum extends backv?ards toward the one on the short trans- 

 verse process until the two nearly meet. The expanded ribs overlap each other from 

 before backwards, forming a strong protection for the thoracic region (plate 8, fig. i). 



Fig. 15. — a. Third, fourth, and fifth ribs of Diadecies phaitolinus. x J. No. 4684 Am. Mui. 

 &, heads of three dorsal ribs of same specimen. 



Posterior to the fifth, the ribs become normal in form. The sixth is nearly or 

 quite the longest and beyond this the ribs gradually shorten. The tuberculum and 

 capitulum are united in a single facet, but in no place is the face on the end of the rib 

 as long as the face on the transverse process; the lower end of the rib face was 

 attached to the transverse process by cartilage. 



Overlying the sixth, seventh, and eighth ribs were thin, elongate dermal plates 

 which overlapped each other from before backwards and carried the protection of 

 the thoracic region backward as far as the distal end of the scapula (plate 5, fig. 5). 

 In the previous description of Diadectes by the author (13) it was stated that there 

 were five plates overlying the anterior ribs. This was an error due to the condition 

 of the specimen in which the scapula overlay the ribs, and the edges of the expanded 

 fourth and fifth ribs were mistaken for the edges of plates. Later, the scapula was 

 removed from one side and the true condition made out. The condition of the ribs 

 and the overlying plates is now known from two specimens. 



In connection with the plates overlying the ribs .should be mentioned the con- 

 dition of the neural spines. In all specimens of Diadectes the upper ends of the 

 spines are expanded and rugose, suggesting very strongly the presence of a row of 

 dermal plates down in the middle of the back. The spines appear much the same 

 as those of Pareiasaurus, in which such a row of plates is known to occur. 



