48 ROYAL SOCIETY OF CANADA 



posterior cingulum and outwardly abuts against the ectoloph; it is 

 of fair size. In the ectoloph the mesostyle is conspicuous, the parastyle 

 is flattened and connects with the protoloph, and the ribs are faintly 

 shown. There is no trace of an internal cingulum. 



Mr. Gidley has drawn the writer's attention to the interesting fact 

 that the protocone in these teeth " is peculiar in having the slope of 

 its anterior face about equal to that of its posterior face,' while in 

 M. hairdi and all the middle and upper Oligocène horses the anterior 

 face of the protocone is always much more abrupt than the posterior 

 one." 



A right upper molar, presumably the third, arbitrarily associated 

 with the foregoing, is considerably larger than the corresponding tooth 

 of 'M. hairdi. A flatness of the ectoloph is principally noticeable, as 

 well as the smallness of the hypocone, otherwise its characters are very 

 similar to those of the left upper second molar already mentioned. 



Mesohippus brachystylus, Osborn. 

 Plate II, fig. 5. 



4i/i left upper premolar, worn {figure 5). 

 Measurement : 



p* a. p. -0125. 



A fourth left upper premolar is referred to this species. The 

 internal cingulum is slightly more accentuated than in the corresponding 

 tooth of the type, but the general proportions and the size seem to be 

 the same. In the Cypress hills specimen the greater part of the ecto- 

 loph is unfortunately missing, but enough remains of its anterior end 

 to show that the parastyle was rounded and of a relatively large size. 

 There are certain slight differences of detail to be noticed but nothing 

 apparently of importance. 



The type of M. hrachystylus is from the Upper Oligocène, Leptau- 

 chenia beds of the Cheyenne river, South Dakota, U.S.A. 



Mesohippus stexolophus, sp. nov. 

 Plate II, figs. 6, 6a and 6b. 



Is^ left upper molar, unworn. 



3rd rigid upper molar, unworn {figures 6, Qa, 6&). 

 Measurements : 

 m^ a. p. -014. 

 m^ a. p. -0125, ly tr. -015. 



These teeth represent an apparently undescribed species of Meso- 

 hippus larger than M. hrachystylus, but resembling it in some particu- 



