THE VERTEBRAL COLUMN 49 



The heavy parapophyses which support the pubic peduncle arise on the side of centrum II, but 

 bear equally on II and III in the form of a triangular web of bone which covers the entire length 

 of each vertebra. The second, third, and fourth parapophyses arise at the junctions of centra III 

 to VI inclusive. The second and third sweep backward to join the acetabular bar while the fourth 

 buttresses the ischiac peduncle and is perpendicular to the axis of the sacrum. This is especially true 



Y.P.M20I5 



Fig. 18. — Sacrum and ilia of Monoclonius {Centrosaurus) flexus. Ventral view, % natural size, a, anterior; f, pos- 

 terior; acb, acetabulum; ds, dorso-sacral vertebra; /'/.ilium; if, ischiac peduncle; far, parapophysis ; ff, pubic peduncle; 

 r, rib of dorso-sacral vertebra; I-X, sacral vertebrae. 



of the right half of the sacrum, as Figure 18 shows. In all of this, the Yale specimen agrees essen- 

 tially with M. crassus and to a less extent with Triceratofs. In the last, the form of the first 

 parapophyses differs in that they arise almost entirely from centrum II and do not have the tri- 

 angular shape as seen from below, being more nearly cylindrical and somewhat dilated distally. The 

 parapophyses of V to VII in Centrosaurus arise opposite the junctions of centra VI to IX. Number V 

 is narrow fore and aft and forms a triangular buttress to the diapophysis. Number VI is somewhat 

 less deep but wider, while number VII is the widest. Parapophyses of VII to IX are broadened 

 distally and fuse with the ilia. That of sacral X arises apparently in the middle of the centrum instead 

 of at the junction with the one before, and does not reach the ilium. This is in agreement with 

 Trkeratops. The acetabular bar, formed by the fusion of parapophyses 1 to IV, unites with the 

 ilium, but with a distinct suture, and forms part of the acetabulum. Both pubic and ischiac peduncles 

 are thus formed in part by the sacral and in part by the iliac elements. 



Caudals 



Except for a few proximal vertebrae and other scattered centra, the caudal series and their 

 chevrons in the Yale specimen of Centrosaurus have been restored from the type of C. nasicornus of 

 the American Museum. Brown's description of the latter follows: "The caudal series is complete 



