rangino^ in North America from the Bridger to the W hite Ri\ er, 

 in which a rimning type developed. Here tlie manus is tetra- 

 dactyl, the pes tridactyl, and as Osborn ' savs : - Tridact) lism is 

 rapidly acquired with a tendency to monodactylism in the lower 

 Oligocene." They strongly suggest the primitive horses in 

 general contour. 



The Equidae are too well known to require more than a brief 

 review. The pentadactyl ancestral form is as yet undiscovered 

 and must be looked for in the Cretacic, for in the lower Eocene 

 there appears Eohippus with a tetradactyl manus, the pollex 

 being represented by a spHnt, and a tridactyl, elongated pes 

 which bears a splint of digit V. The other Eocene horses 

 exhibit the same stage of digital reduction as in Eohippus ; but 

 Mesohippus of the middle and upper Oligocene is tridactyl in 

 the manus as well as in the pes, the fifth digit of the former 

 showing a sphnt-like metatarsal, digit I being vestigial. Here 

 all of the digits are functional the laterals finally losing their 

 contact with the ground in Merychippus of the middle' Miocene 

 and in Neohipparion of the upper Miocene, a beautiful specimen 

 of which has lately been added to the American Museum col- 

 lection. Hypohippus of the middle Miocene with subfunctional 

 lateral digits and, in the manus, the vestige of metacarpal V is 

 an instance of arrested evolution owing probably to marsh 

 dwelling habits which necessitated a spreading foot. 



Finally the monodactyl type of the Pleistocene and Recent is 

 represented by the genus Equus in which digits I and V are 

 reversional and digits II and IV vestigial, being represented by 

 the metapodial splints alone. 



The Equidae are curiously paralleled in foot reduction by the 

 South American Litopterna in which the tridactyl condition 

 with functionless lateral digits is shown in Proterotherium from 

 the Santa Cruz formation, Lower Tertiary of Patagonia. This 

 creature seems to parallel Merychippus, the main distinctions 

 being that the former has rather more slender phalanges in the 

 middle digit while those of the lateral digits are proportionately 



