46 



THE CERATOPSIA 



Dorsal XI 



(Fig. 16) 



This vertebra shows the following characteristics: centrum constricted at mid-length; pleuro- 

 coeles fairly deep; anterior face nearly plane, posterior one deeply concave; prezygapophyseal facets 

 confluent and nearly horizontal; postzygapophyseal facets forming an angle of 90° with each other. 

 The latter are well below the former, and well behind the centrum as in the preceding vertebrae. 

 The capitular facets are situated about one-third of the way out on the anterior edge of the zygapo- 

 physes. From them a groove extends outward on the anterior edge to the end of the diapophysis for 

 contact with the rib. The spinous process is plain, slightly tapering, and slanting well backward. 



Y.P.M. 2015 



Fig. 16. — Dorsal XI of Monoclonius (Centrosaurus) flexus. Anterior and right lateral views, Y\ natural size, az, 

 anterior zygapophysis; d, diapophysis; //, capitular facet; n, neural canal; f, pleurocoele; fz, posterior zygapophysis ; 

 s, neural spine; t, tubercular facet. 



Compared with Triceratofs, the vertebra in question differs in the shape of the centrum, when viewed 

 from the end, that in Centrosaurus being more nearly circular and that in Triceratofs somewhat 

 pear-shaped. In Triceratofs there is less slope to the neural arch and spinous process. The general 

 proportions of the zygapophyses and spinous process are somewhat similar, but the surface for con- 

 tact with the rib between capitular and tubercular facets is much less marked in Triceratofs. The 

 latter has no pleurocoele. 



Measurements of Dorsal XI 



Centrum, length at top 67.5 mm. 



Centrum, width of anterior face 100.0 



Centrum, height of anterior face 78.0 



Width over diapophyses 253.0 



Width over capitular facets 115.0 



Length of spinous process 140.0 



Neural canal, height of anterior end 16.0) .„ , .. 



XT , i • , , r ■ j in i C ill preserved outline 



Neural canal, width of anterior end Ay . 5 ) 



Sacrum 



(Fig. 17) 



The sacrum in Centrosaurus, as in all of the later Ceratopsia, consists of ten coalesced vertebrae. 

 To these is added a dorso-sacral, coossified with the sacrum above but free below. The spinous 

 processes, including that of the dorso-sacral, have become fused into a solid bar, the original limits of 



