60 THE CERATOPSIA 



ently, the first from side to side, the second from front to rear. In Centrosaurus cutleri, Brown 32 

 describes the femur as "flattened but the general form is well preserved, displaying a long, straight 

 shaft with a large head that rises slightly above the level of the great trochanter. The lesser 

 trochanter is separated from the great trochanter by a narrow cleft and terminates slightly below its 

 summit. The fourth trochanter is a faint elevated ridge about six inches long that terminates a little 

 below the middle of the shaft. The distal end of the femur curves inward from the line of the 

 shaft and the inner condyle appears to be larger than the outer condyle, a condition which if true 

 is the reverse of Triceratops. The crushed femora in the type specimen of C. nasicornus fail to settle 

 this point." In the Yale femora the agreement with C. cutleri is essential except that the shaft is not 

 flattened and the fourth trochanter seems to be lower down than in C. cutleri. The length of each 

 is about the same as is the ratio of femoral to tibial length. In the femur under discussion, that is, 

 the left, the inner condyle is eroded and crushed in such a way that the outer one is much the larger, 

 agreeing with Triceratops but the reverse of Brown's description of C. cutleri, whereas in the right 

 femur the restorers have made the inner condyle the larger, agreeing with C. cutleri. Which of 

 these is correct is not apparent. 



Dimensions of Femur 



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

 Outside length from top of great trochan- 

 ter to bottom of external condyle 789 mm. 740 mm. 800 mm. 713 mm. 



Girth, mid-shaft 320 



Width, mid-shaft 85 



Fore and aft diameter of shaft 97.5 



Width of proximal end 180 



Width, fore and aft fat great trochanter) 167 



Width, fore and aft (at external condyle) 189 



Tibia 



(Fig. 28) 



The tibia differs from that of Triceratops in being more slender throughout, and having rela- 

 tively less expansion at the ends. In both genera the distal end of the tibia is widened transversely, 

 there being no antero-posterior increase in the dimensions of the bone. Apparently lacking in 

 Triceratops, the calcaneum seems to be present in Centrosaurus, Chasmosaurus, and Anchiceratops 

 although not always distinguishable from the astragalus. Between them, these two bones cover the 

 entire distal end of the tibia in these earlier genera. In Triceratops, on the other hand, the cal- 

 caneum has either been replaced by, or absorbed into, a heavy, downwardly directed process of the 

 tibia comparable to that of the inner side but, unlike the former, descending somewhat below the 

 level of the astragalus, with which it is in close contact on the latter's external side. This, Hatcher 

 says, 33 "may be regarded as having performed the same functions as did the external malleolus or 

 distal end of the fibula in the mammalia." 



The proportionate lengths of tibia and femur vary with the species. In Triceratops prorsus 

 (mounted specimen in U. S. Nat. Mus.) the femur is half again as long as the tibia. In Centrosaurus 

 nasicornus and Monoclonius crassus the tibia is relatively longer, while in C. cutleri^ and the Yale 

 specimen of C. flexus the proportions are similar to those of Triceratops. Brown believes this rela- 

 tive size to be of at least specific value in this family; Nopsca, 34 however, would consider such varia- 

 tion as is here indicated to be sexual. In any case, these proportions form an additional point of 

 similarity between the Yale specimen and the partial skeleton forming the type of C. cutleri. 



32 Brown, B., 1917, p. 304. 



33 Hatcher, Marsh, Lull, 1907, p. 62. 



34 Nopsca, F., 1929, B. 



