PLATE 31 



PLIOHIPPUS AND HIPPARION FROM SOUTH DAKOTA AND NEBRASKA. 



UPPER AND LOWER MOLARS, UNWORN, WITHOUT CEMENT, TO SHOW PARALLELISM AND DIFFERENCES IN CONSTRUCTION OF 



THE CROWNS. 



All figures natural size. 



Note the curvature of the upper molar in Pliohippus (3, c, d) as stronger than in Hipparion (1, c, d) ; also the separation 

 of the protocone to base of tooth in Hipparion (1, b, c, d), prominence of the parastylid in Hipparion (2, a, c), and the com- 

 plete separation of metastylid from metaconid (2, b). 



Little White River, S. D. Lower Pliocene 1, 2. Hipparion whitncyi Gidley, referred specimen, Am. Mus. 9817. 



1, Upper molar; a, external, b, internal, c, anterior, d, posterior, 



and, 1, crown views; 2, upper molar, same views. 

 Snake Creek, Sioux Co., Nebr. 3. Pliohippus supremus Leidy, referred specimen, Am. Mus. 



10827. Upper molar. Same views as in figs. 1, 2. 

 " " " " -i. Pliohippus supremus Leidy, referred specimen, Am. Mus. 



10848. Lower molar. Same views as in figs. 1, 2. 



284 



