144 



osborn: OLIGOCENE, MIOCENE, PLIOCENE EQUID.E. 



Characters. — (Cope, 1889, Osborn, 1918) (1) In dimensions the type of this species is about equal to " Hippothcrium " 

 speciosum [a referred specimen]; (2) "The other six (with one exception) are less worn, and present a less complex folding 

 of the enamel plates [than in Protohippus retrusus]" ; (3) protocone of the same flattened form as in P. retrusus but con- 

 nected with protoconule by a narrow isthmus; (4) separated in two of the teeth from the metaconule; (5) p 2 presenting a 

 complete fusion; (6) approaching nearer to Equus than any known species of Equus or Hippidiurn. 



Gidley (1907, p. 908) observes that this species, like P. retrusus, is of doubtful reference and was not well characterized 

 by Cope. Its greater size separates it from P. retrusus, although its characters, so far as they can be made out, seem to 

 class it in the same group. Matthew (1913) observes that the imperfect teeth which form the type are closely allied to P. 

 perditus but are somewhat shorter-crowned, with larger enamel lakes, and perfectly simple enamel borders. The referred 

 hind limb and foot bones are exceptionally long and slender. 



Protohippus perditus secundus mut. nov. 

 Plate 22.1. Text Fig. 116, 116a. 



Horizon and locality. — Republican River beds, Driftwood Creek, western Nebraska, Peraceras zone, Upper Miocene 

 or Lower Pliocene. Type collected by R. S. Hill. 



Type. — Amer. Mus. Cope Coll. 8340, a series of upper grinding teeth of the right side, p 2 -m 3 , associated with pelvis, 

 hind limb, and foot bones. 



Type figure — Plate 22.1, text Fig. 116, 110a. 



Characters. — (1) Molar-premolar series (text Fig. 110a) somewhat exceeding in length that of the type of P. 

 perditus. (2) Hind limb elongate; (3) metatarsal III slender, elongate, laterally compressed. 



This animal is apparently a mutation beyond the typical P. perditus. The discovery of the foot bones of this form 

 and of a referred P. placidus reveals the very important feature of the slender, elongate, laterally compressed central meta- 

 podials and the abbreviated lateral digits; anisotridactyly in contrast to the monodactyly of the contemporary Pliohippus. 



