THK MAMMALIA OK THE UINTA FOHMATION. 545 



vaUdns (Marsh), there is a rciiinaiit ol' int. i.* In the Eocene genera the separate 

 elements are ver}' nairow and vortifaily elougatc ; hilor the elements become broad 

 and spreading. A primitive and persistent feature is the triangular cuboidal facet 

 upon the astragalus (see Diag. 10). Another marked characteristic, early devel- 

 oped, is the reduction in size of the ''inferior" astragalo-calcaneal facet and its wide 

 separation from the " sustentacular." An importaut equine mark is the depression or 

 fossa in the astragalo-navicular facets ; this is a slight valley (r) in the Bridger spe- 

 cies of Hyracotherium, which is variable in size but constantly increasing in the 

 ascending series, terminating in an extensive fossa, in Equus. 



There are, moreover, a number of minor tarsal features not always persistent but 

 of great value in diagnosis; the three astragalo-calcaneal facets (c, c", c'") are always 

 separate, the trochlear groove is deep and sharp. In many Eocene species the navicu- 

 lar is inserted anteriorly between the cuboid and astragalus and thus develoiJs a dis- 

 tinct calcaneal facet; this feature does not persist. In all Eocene specimens the cal- 

 caneum has a distinct pit for the fibula, in extreme flexion ; this is usually facetted; 

 it reappears in Mesolitppu^ and sometimes persists in AnrhilherinmA- 



Tiic metatarsal aiticulations pass through three distinct types. In the Eocene 

 genera we find the "alternating" type, in the Miocene the " plane serial," replaced 

 later by the equine type. 



Ilijracotherium venUcolum, Pliolophus and Epihippus, the Eocene genera, may be 

 considered together. The first species from the Wind River is distinguished by the 

 absence of the fossa in the astragalo-navicular facet (Cope, ''Tcrt. Verl.."' p. 045); this 

 fossa is more or less distinct in five small and two large astragali {P. pHmilus and 

 P. major), in the Princeton collection, from the overlying Bridger, and very distinct 

 in the P. (Tri/ilopus) amaroram o( the still more recent Washakie (Co|)eColl.). 

 The fibulo-calcaneal pit is present throughout, but is not invariably facetted. The 

 naviculo-calcaneal facet is not present in JI. venticoluiu ; it appears in P. inajor and 

 P. amarormii, but not apparently in P. pwniliis. The highly coinpi-cssed tridactyl 

 character of the foot is retained throughout. The mts. iii and iv articulate exclu- 

 sively with the ectocnnciform and cuboid respectively. Mt. ii articulates proxinially 

 with the mesocuneiform, also laterally with the culo- ami (•cldcuntifniin-i. These 

 relations are seen in //. renticohiin and in P. rnnjor. In /'. (unaronnn, mt. iv articu- 

 lates laterally with the ectocnnciform. This sjKcies is also distinguished by the 

 flatteninsr of the navicular and cuneiforms and relative shortening of tin- cuboid. 



•Mnrah, Am. .lour. He. and Arts, Nov , 1H70, |>. 401. 



f This Tacft Ls well mnrkcd in a calpaiiciim Inlely received in cxcli.ingc fron) the Pi»ri.H .Mii.iciim, nmrked .1. mire- 

 lianentt, from Sansnn. Soc Olag. 10. 



