STRUCTURE AND RELATIONSHIPS OF OPISTHOCCLIAN DINOSAURS. 191 
out the greater part of the series are directly opposing. However, in 
the first vertebra they face upward as'well as inward and have their 
bases excavated by deep lateral cavities. The post-zygapophyses 
take the form of vertical plates borne upon the posterior angles of the 
neural spine. They are supported from above by a vertical continu- 
ation of the lateral spinous plate. In the anterior caudals there is 
also a lateral brace which springs from the diapophysial element of 
the lateral plate. | 





Fic. 18. Anterior view of chevrons, showing gradation in structure. 
The caudal chevrons are of three types, the closed arch, the open 
arch, or transitional type, and the double arch. (Fig. 18.) Of these 
the latter two appear far back in the series where the vertebre are 
much reduced in size. The closed arch may therefore be considered 
as the most Characteristic type. Many of the chevrons found with 
this specimen were so displaced that their position cannot be accurately 
determined. The presence of a short, stout chevron imbedded in the 
matrix beneath the first caudal vertebra suggests that all of the anterior 
caudal vertebra may have been chevron-bearing. 
