58 



THE CERATOPSIA 



Fig. 26. — Right pubis and ischium of Monoclonius (Centrosaurus) flexus, viewed from the inner surface, 1/6 natural size. 

 i, ischium; il, facet for articulation with ilium; f, pubis; fb, facet for articulation with pubis; ff, postpubis. 



Ischium 



(Fig. 26) 



The proximal halves of both ischia are present in the Yale specimen, with clean-cut breaks. 

 As preserved, they seem to agree more nearly with C. cutleri and Monoclonius crassus in being 

 straighter than in C. nasicornus. In the latter, the shaft, viewed from the side, has an almost uniform 

 curve, increasing somewhat distally, whereas in C. cutleri the bone is straighter until the extreme end 

 is reached, when the curve is very marked and the end dilated, recalling the hammerhead-like ter- 

 mination seen in the crested trachodons of the Belly River. 30 Chasmosaurus (PI. XIV, B), as 

 mounted in Ottawa, also possesses this type of ischia, while C. nasicornus is more like Triceratofs 

 in this respect. Proximally, there is little distinction between the ischia belonging to the several 

 Centrosaurus species. The surface for articulation with the pubis is smaller and lighter than the 

 iliac facet, which is continued into a rugose area on the outer aspect of the bone. There is a 

 corresponding rugosity on the outer aspect of the pubis. 



Dimensions of Ischium 



C. flexus Y.P.M. C. cutleri C. nasicornus 



Width across proximal end 250.0 mm. mm. mm. 



Least diameter of shaft 35.0 



Greatest diameter of shaft 57.5 



Iliac peduncle to end (measured on curve), estimated .... 770.0 800.0 740.0 



30 This feature Nopsca (1929 B) considered indicative of the male sex, those which did not possess it being females. 

 Unfortunately for his argument, however, all known specimens of the Lance genus Trachodon would be females! 



