532 THE MAMMALIA OP THE UINTA FORMATION. 



II. The main features of the mamis and pes in the Hyracoidea, Amblypoda, 

 Proboseidia, Toxodontia, and in Macrauehenia. 



III. The chief and minor characteristics of the manus and pes in the various series 



of Perissodactyla. 



IV. The taxonomic value of the primitive and secondary articulations in the manus 



and pes. 

 V. The laws of modification of foot structure. 



I. THE PRIMITIVE FOOT. 



The more recent types are connected with the remarkably simplified feet of the 

 Puerco ungulates by Phenacodm ; without this link it would be presumption to 

 describe the feet of such genera as Periptychus, Uctoconus, and Memscotherium, as 

 ungulate, since, excepting in the terminal phalanges, they resemble the plantigrade 

 carnivore foot far more closely than those of any living ungulate — possibly excepting 

 Hyrax. 



The pes of the Puerco Penptychus and Pantolamhda are fully known. The 

 former corresponds so closely with that of Urf<us, we may anticipate a similar homo- 

 logy in the manus, especially as the entire structure of the upper portion of the fore 

 limb is bear-like. Our actual knowledge of the primitive taxeopod manus is at 

 present inferential from Plienacodus in comjjarison with the more recent types in 

 which taxeopod characteristics persist — as in most cases we can readily detect and 

 eliminate secondary structures. 



General cliaraciers. The primitive manus and pes of the lowest Eocene ungu- 

 lates had five spreading digits bearing spatulate terminal phalanges marked by a me- 

 dian groove. The first and fifth digits were shoit. The separate elements of the 

 podium and metapodium were articulated serially, the only interlocking joints being, 

 as in all mammalia, beneath the scaphoid and unciform, cuboid and navicular. In 

 standing, the entire sole rested upon the ground, the foot forming a right angle with 

 the lower leg; the limited astragalo-tibial facets in all the Puerco forms leave no 

 doubt upon this point, and we have a persistent example of this type in Hyrax. 



The pp:s. In the pes both the tibia and fibula wei'e supported upon the astragalus; 

 a f i b u 1 o - c a 1 c a n e a I facet is frequently assigned as one of the characteristics of 

 the primitive ungulate foot,* from the fact of its appearance in so many difterent 

 phyla. Among the primitive Ungulata this facet has thus far been observed only in 

 Meniscotherium, it is wanting in all the other Condylarthra. (1) In this genus it has 



* See Marsh, Diiiorcnila, C'li:n:icli'rs of llic J'rotiingiihild, \). 171. 



