THE SKELETON IN MAMMALIA. THE PES. 525 



of Subungulates in having the bones arranged in lines with 

 little or no interlocking. The calcaneum resembles that of 

 Artiodactyles in having a small facet for articulation with the 

 fibula. Tapirus (fig. 110, A), Rhinoceros (fig. 110, B) and 

 Titanotherium have a short and broad foot with the usual 

 tarsal bones and three well-developed digits, a number never- 

 exceeded by any Perissodactyle. From this tridactylate limb 

 a series of stages is exhibited by various extinct forms leading 

 gradually to the condition met with in the Horse (fig. 110, D) 

 in which the third toe is greatly developed, while the second 

 and fourth are reduced to slender metatarsals attached to the 

 proximal half of the third metatarsal. 



In Chalicotherium and Agriochoerus the pes has the same 

 abnormal characters as the manus, the digits being clawed and 

 the ungual phalanges in Chalicotherium deeply cleft. 



In the SUBUNGULATA the pes is sometimes plantigrade and 

 pentedactylate, the cuboid sometimes does not articulate with 

 the astragalus, and the tarsal bones sometimes do not inter- 

 lock. 



In Typotherium (TOXODONTIA) the hallux is absent and the 

 other four digits are well developed ; in Toxodon and Nesodon 

 the pes is tridactylate. The tarsal bones have the regular Sub- 

 ungulate arrangement, the cuboid not articulating with the 

 astragalus. The calcaneum articulates with the fibula as in 

 Artiodactyles. The astragalus in most forms, but not in 

 Astrapotherium, resembles that of the Ungulata vera in having 

 a grooved proximal surface. 



In Phenacodus (CONDYLARTHRA) the tarsus is very little 

 modified, five digits are present, the first and fifth being small 

 and not reaching the ground. 



In Procavia only the three middle digits are present with 

 a vestige of the fifth metacarpal. 



In the AMBLYPODA the pes (fig. 108) is very short and 

 broad, all five digits are functional, and at any rate in 

 Coryphodon plantigrade, the hallux being the smallest. The 



