62 THE CERATOPSIA 



Pes 



Of the tarsalia in the Ceratopsia, but four elements have been discovered, two in the proximal 

 row the astragalus and calcaneum, and two 36 in the distal row of which the larger represents tarsal 

 IV. Hatcher knew only the astragalus among the tarsalia, although he assumed that others were 

 present. His description of the astragalus of Tricerato-ps was brief; he says, "It was closely applied 

 and early became coossified with the tibia, covering over the internal two-thirds of the distal extremity 

 of that bone." In Centrosaurus, on the other hand, the astragalus covered about three-fourths of 

 the end of the tibia. The astragalus is thick, with a convex distal surface and a concave proximal 

 one. The posterior border is thin and rises higher on the surface of the tibia than does the thicker 

 anterior margin. There is no ascending process, as in certain other groups of dinosaurs. 



Y.PM. 2015 



F, G- 29. — Anterior view of left hind foot of Monoclottius (Centrosaurus) flexus, 1/6 natural size, m, metatarsal; 



p 1 -? 5 , phalanges; digits I-V; t, tarsals. 



The calcaneum is a much smaller bone which in the Yale specimen is so closely fused with the 

 astragalus that its presence as a separate element would hardly be detected. It has somewhat the 

 form of the astragalus but does not rise so high on the posterior surface of the tibia. Between them, 

 these two bones cover the entire distal end of the tibia and form a very perfect surface for articula- 

 tion with the remainder of the tarsus which, however, must have been largely cartilaginous, as the 

 small distal tarsals do not begin to complete this segment of the limb. Of the distal tarsalia the 

 larger, according to Brown, articulated with the fourth metatarsal. The position of the other is 

 doubtful; possibly it lay above metatarsal III. 



The left foot (Fig. 29) is practically entire in the Yale specimen, only the fifth metatarsal 

 and the first and fourth ungual phalanges being artificial. Four functional digits are present, all 

 of which are provided with ungual phalanges, as against three in the manus. The fifth digit is 

 vestigial and is represented by the proximal part of the metatarsal only. Thus, the phalangeal 

 formula is 2, 3, 4, 5, 0, which is normal for a predentate dinosaur. 



Metatarsal I is short, heavy, and rather irregular, the proximal end somewhat triangular, but 

 with one of the angles rounded. The articular surface is slightly concave. There is a rugose area 



36 See, however, p. 69, Chasmosaurus. 



