1910.] 



Characters of the Stem Perissodadyls. 



393 



The carpus (see also p. 450) of the ancestral Perissodactyl diflFered sharply 

 from that of Phenacodus in that it was not altogether serial but partly dis- 

 placed or interlocking in type: i. e., with the scapho-centrale touching the 

 small magnum, the lunar resting not only on the nuxgnum but 'partly on the 

 unciform (see j). 446). This condition is shown in all the known manus of 

 Eocene Perissodactyls, representing the earlier types of each family. Form- 

 erly when Phenacodus itself was 

 held to be ancestral to the Perisso- 

 dactyls it was easy to believe that 

 the so called displaced type of 

 carpus in the early Perissodactyls 

 was secontlary. But evidence was 

 adduced by Matthew (see p. 446 

 below) that the serial carpus of 

 Phenacodus is itself secondary, 

 since the manus of Euprotogonia 

 retains the lunar-unciform contact; 

 and in the present work it is shown 

 (pp. 442-446) that the lunar-unci- 

 form and scapho-centrale-magnum 

 contacts are characteristic of many 

 of the lowest pentadactyl unguicu- 

 lates and persist into the orders 

 Creodonta, Amblypoda, Primates, 

 Artiodactyla, Litopterna, etc. 



It is not intended to imply that 

 the carpus in the ancestral Perisso- 

 dactyl was of the fully "displaced" 

 type (p. 451). In the Lower Eocene 

 "Lophiodont" Heptodon calciculus 

 (Fig. 23) the lunar-unciform con- 

 tact, while very considerable in 

 front view, is narrower in rear view; 

 and in the Rhinoceros line it is easy 

 to follow the progressive broadening of the scapho-centrale-magnum and 

 lunar-unciform contacts, through Plyrachyus and Ccenopus to the widely 

 displaced type in Rhinoceros. 



The magnum of the ancestral Perissodactyl in front view was small and 

 compressed but in side view was large and shaped like a shoe, the toe point- 

 ing downward, the sole backward (Fig. 24). The strong flexor carpi 

 muscles were attached to the sole of this shoe, while the rand or part above 



Fig. 23. Left manus of a Lower Eocene 

 Tapiroid (Lophiodont) Heptodon calciculus Cope 

 (Amer. Mus. No. 294). Ce., hook of the scaph- 

 oid, formed by the coalesced centrale. In the 

 back view the lunar is seen to rest almost 

 equally on the magnum and unciform X §. 



