37 



upper anterior surface of the style (figures 19 and 20). The cross crests are narrow in propor- 

 tion to their height, a feature suggested in the name that has been given to the species. 



Mesohippus planidens, Lambe. 

 Plate III, fig. 21. 



Mesohippus planidens, Lambe, 1905. Fossil horses of the Oligocene of the Cypress hills, 

 Assiniboia ; Trans. Royal Soc. of Canada, second series, vol. XI, section IV, p. 49, pi. 

 II, fig. 7. ' 



1st and 2nd left upper molars, worn. Type. 

 Measurements in mm. 



m l m 2 31 



m l a. p 15 by tr 17-8 



These teeth indicate a species of about the size of M. intermedins, Osborn, from the Upper 

 Oligocene, Protoceras beds of South Dakota, but smaller than M. validus, Osborn, from the 

 same horizon and state. They differ from those of M. intermedins in the greater obliquity 

 of the protoloph and metaloph, in which respect they resemble those of 31. obliquidens, 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 inconspicuous. 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 develops a rudimentary crochet. The protocone and hypo- 

 cone are not so elevated as the ectoloph. 



The specific name has reference to the flattened condition of the ectoloph. 



Mesohippus assinibotensis, Lambe. 



Plate III, figs. 22, 23 and 21. 



Mesohippus assiniboiensis, Lambe, 1905. Fossil horses of the Oligocene of the Cypress hills, 

 Assiniboia; Trans. Royal Soc. of Canada, second series, vol. XI, section IV, p. 50, pi. 

 II, figs. 8, 8a and 86. 



2nd right upper premolar, unworn. Type. 

 Measurements in mm. ~ 



p 2 a. p 185 by tr 17, height of tritocone. . . .10'5. 



height of teta.rtocone. . . .8. 

 This species is larger than M. intermedius, Osborn and Wortman, and apparently than 

 M. validus, Osborn, from the Leptaucheniabeds ot S. Dakota. It resembles M. brachystylus, 

 Osborn, from the Leptauchenia beds of S. Dakota, in the great development of the parastyle, 

 which is, however, more distinctly separated in the Cypress Hills species. 



In the above tooth (p 2 ) the antero-posterior diameter is greatly increased by the separ- 

 ation and large size of the parastyle. The cross crests are short, steep-sided and set almost at 

 right angles to the ectoloph. The two inner cusps (deuterocone and tetartocone) are strongly 



