THE SKELETON IN MAMMALIA. THE MANUS. 509 



(fig. 106, A) and Hyracotherium the fifth digit is fully de- 

 veloped but is scarcely functional. In Titanotherium (Er on- 

 tops] (fig. 106, B) it is nearly as well developed as any of the 

 others, and there is little or no difference between the relative 

 development of the third and fourth digits. 



The Chalicotheriidae l , though distinctly Perissodactyles in 

 various respects such as their cervical vertebrae and teeth, 

 differ not only from all other Perissodactyles, but from almost 

 all other Ungulates, in the very abnormal character of their 

 manus. For while the carpus and metacarpus are like those 

 of ordinary Perissodactyles, the phalanges resemble those of 

 Edentates, each second phalanx having a strongly developed 

 trochlea, and each distal one being curved, pointed and deeply 

 cleft at its termination (fig. 106, C). 



The Macraucheniidae, while agreeing with Perissodactyles 

 in having only three digits, with the limb symmetrical about 

 a line drawn through the middle of the third, have a carpus 

 which approaches closely to the subungulate condition, the 

 magnum articulating regularly with the lunar, and only to a 

 slight extent with the scaphoid. 



In the SUBUNGULATA the manus sometimes has five func- 

 tional digits, and a considerable part of it rests on the ground 

 in walking. The bones of the carpus retain their primitive 

 relation to one another, the magnum articulating with the 

 lunar, but not with the scaphoid. This character does not 

 however hold in the Toxodontia, for in most of the animals 

 belonging to this group the magnum does articulate with the 

 scaphoid. The corner of the scaphoid just reaches the magnum 

 also in Amblypoda. 



As far as is known the TOXODONTIA generally have three, 

 sometimes five digits to the manus, and the third is symme- 

 trical in itself a Perissodactyloid feature. 



In Phenacodus (fig. 107, B) (CONDYLARTIIRA) all five digits 



1 See H. F. Osborn, Clialicotherium and Macrotherium, Amer. Natural. 

 188991-92. 



