THF VFRTFRRAL COLUMN 



47 



the individual processes being for the most part indistinguishable in the present condition of the Yale 

 specimen. The spinous processes are low and their superior margin is essentially straight throughout 

 the first five sacrals, beyond which the individual summits are restored in such a way as to give a 

 saw-tooth outline, as viewed from the side. Whether or not this restoration is correct is difficult to 

 determine, for in the C. nasicornus sacrum the spines are entirely obscured by the ossified tendons; 



Y.P.M. 2015 



F, c i7_ — Sacrum and ilia of Monoclonius (Centrosaurui) flexus. Dorsal view, •/£ natural size, a, anterior; f, posterior; 

 ax, anterior zygapophysis of dorso-sacral vertebra; d, diapophysis; il, ilium. 



in the M. crassus type the summits of the spines are broken away; and in C. cutleri the posterior 

 summits are separate, but in line. In the Yale specimen, the zygapophyses and spinous process of 

 the last sacral are distinct, the latter being isolated. The zygapophyses are well indicated only 

 between the last two sacrals; otherwise, except for the prezygapophyses of the dorso-sacral, they 

 are thoroughly fused. The diapophyses slope slightly upward (from the horizontal) from the 

 neural arch outward, the angle of slope being highest at sacral IV, and least at X. The diapophyses 

 of the dorso-sacral are heavier than the two immediately following and the anterior edge is deflected 

 sharply downward. Diapophyses of sacrals I and II are rather narrow and flat, but reach the blade 

 of the ilium, whereas that of sacral III is wider and heavily buttressed below to meet the pubic 

 peduncle. This lower buttress is the parapophysis, which in Triceratops arises from the third sacral 

 although bearing also against the second, is deflected backward, and is not fused with the diapophysis 

 above. In Centrosaurus, the parapophysis seems to arise from the second sacral and sweeps backward 

 so that the pubic peduncle is opposite the centrum of sacral III. Contrary to Triceratops, para- and 

 diapophyses are fused in sacrals III-IX. The type of M. crassus agrees with the Yale specimen in 

 this respect. Dia- and parapophyses of III-VI form the acetabular bar which abuts against the 

 acetabulum, agreeing with Tricerato-ps. The parapophyses in the rear of the acetabular bar are thin 



