394 



Bulletin American Museum of Natural History. [Vol. XXVII, 



the heel was keeled for the support of the lunar on one side and of the scapho- 

 centrale on the other. These characters of the magnum are strongly marked 

 in all the Lower Eocene Perissodactyls examined and sharply separate the 

 latter from Phenacodus in which the toe of the magnum is at most represented 

 by a blunt low process. Most of the later Perissodactyls retain this "toe" 

 in a more or less modified condition, but in Lophiodon as figured by Deperet 

 (1903) it is wanting. It is also developed to some extent in the x\rtiodactyla 

 (e. g., Oreodon) but is there shaped quite differently. 



The unciform was inclined somewhat upward and this condition in- 



fcx.cr. , 





Fig. 24. Left magnum of two Lower Eocene Perissodactyls, Heptodon calciculus (Cope) 

 Amer. Mus. No. 294, X f and Eotitanops borealis (Cope) Amer. Mus. No. 296. X |. The small 

 end faces anteriorly, the bones are seen from the outer side. 



? ex. c. r., tuberosity for ? extensor carpi radialis. 



?,^. c. r.. " " ? flexor 



ice.), facet for hook of the scapho-centrale. 



(lu.), " " lunar. 



(unc), " " unciform. 



(Ill) " " metacarpal III. 



creased with the broadening of the lunar-unciform contact; the unciform 

 had a prominent process on its postero-external face. 



The lunar was wedge-shaped distally, with an oblique anterior facet for 

 the magnum. The centrale was very early fused with the scaphoid, forming 

 the so called "hook of the scaphoid" in Rhinoceroses. 



The successive overlap of the metacarpals II-IV on the external proximal 

 ends was an inheritance from the Insectivore-Creodont stock and due origi- 

 nally to the fact that the magnum and trapezoid were shallow small bones 

 which caused their attached metacarpals III and II to appear as if thrust 

 up into the carpus above the lower level of the unciform and trapezium. 

 The distal ends of the metacarpals were without keels and were neither 



