[lambe] fossil horses OF THE OLIGOCENE 49 



lars. The differences noticed are: (1) the greater relative size of m' 

 with a more pronounced obliquity of the cross crests in these teeth; 

 (2) the greater length of the metaloph, which in m^ is connected with 

 the ectoloph, and (3) the intimate connection of the hypostyle with both 

 the posterior cingulum and the metastyle. The resemblances are : — 

 (1) somewhat similar general proportions with about the same degree 

 of development of the protoconule and a like suppression of the meta- 

 conule; (2) the parastyle and internal cingulum similarly developed. 

 A special character of M. stenolophus, seen in m^, is the oblique 

 crossing of the parastyle by the external cingulum which rises rapidly 

 from without and appears very distinctly on the upper anterior surface 

 of the style (figures 6a, 6b). The cross crests are narrow in proportion 

 to their height, a feature suggested in the name given to the species. 



Mesohippus planidens, sp. nov. 

 Plate II, fig. 7. 



1st and 2nd left upper molars, worn. 

 Measurements : 



m^ — m^ -031. 



m^ a. p. -015 Uj tr. -0178. 



These teeth indicate a species of about the size of M. intermedium, 

 Osborn and Wortman, from the Upper Oligocène, Protoceras beds of 

 South Dakota, but smaller than M. validus, Osborn, from the same hor- 

 izon and state. They difi'er from those of M. intermedins in the greater 

 obliquity of the protoloph and metaloph in which respect they resemble 

 those of M. ohliquidens, Osborn. The teeth are brachyodont, and are 

 devoid of an internal cingulum. Externally the ectoloph is noticeably flat 

 with only a slight development of the parastyle and mesostyle, the ribs are 

 absent or but feebly indicated, and the metastyle is particularly inconspi- 

 cuous. The hypostyle is of fair size, curved and attached at either end to 

 the posterior cingulum. The cross crests are oblique to the ectoloph, well 

 elevated, and moderately continuous, the metaloph more so than the 

 protoloph. The protoconule slightly interrupts the protoloph and unites 

 with the parastyle. The metaconule scarcely breaks the continuity 

 of the metaloph which is sharply separated from the ectoloph and 

 developes a rudimentary crochet. The protocone and hypocone are not 

 so elevated as the ectoloph. 



The specific name has reference to the flattened condition of the 

 ectoloph. 



Sec. IV., 1905. 4 



