202 



osborn: OLIGOCENE, MIOCENE, PLIOCENE EQUID.E. 



Type figure. — Text Fig. 166 of this Memoir. 



Characters. — (Leidy) (1) Type an animal one-third larger than Hipparion gracile, or the Floridian H. plicatile, equal- 

 ling in size Equus cahallus; (2) the tooth, a second or third upper "molar," is three inches long in its outer curvature, and 

 the worn triturating surface, represented in the accompanying wood cut, measures fifteen lines fore and aft and fourteen 

 lines transversely; (3) the arrangement of the enamel most nearly approximates the condition observed in H. occidentale 

 from our western Tertiary formation ; (4) the inner column, [protocone] of uniform breadth the entire length of the crown, 

 measures half an inch fore and aft and in section is horizontally reniform, i. e. the protocone is elongate anteroposteriorly, 

 convex externally, slightly concave internally. (Osborn, 1918) (5) Deep enamel plications on both anterior and posterior 

 borders of the pre- and postfossettes, double crochet fold in prefossette, and double pli caballin folds opposite protocone. 



Gidley (1907) related this species to his " Neohipparion " group with broad, flattened protocones, and regarded it as a 

 very large and highly specialized animal, perhaps one of the last of the " N eohi pparion " group which may have survived 

 into the Pleistocene, being contemporary with some of the earlier species or Equus, with remains of which the type tooth 

 was found associated. 



Fig. 166a. Type of Hipparion princeps Leidy, U. S. National Mus. 3299, as sectioned by J. W. Gidley, to demon- 

 strate its relationship to if not identity with the paratype of Equus fraternus Leidy. 



Gidley (1918) recently had the H. princeps type sectioned less than a-half inch above its triturating surface, and found 

 that somewhere below this plane the protocone joins the protoconule; and that therefore the only Hipparion character of 

 this type disappears. The type of H. princeps cannot now be distinguished from corresponding teeth of Equus fraternus 

 Leid y, as that species has been redefined by Hay; it agrees in size, proportions, enamsl plication, and general appearance 

 (Gidley, letter to author, Jan. 30, 1918) . 



