36 



A right upper molar, presumably the third, arbitrarily associated with the foregoing, is 

 considerably larger than the corresponding tooth of M. bairdi. 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 III, fig. 17. 



Mesohippus brachystylus, Osborn, 1904. New Oligocene horses ; Bulletin Amer. Mus. Nat. 

 Hist., vol. XX, p. 175, fig. 6. 



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

 Assiniboia ; Trans. Royal Soc. of Canada, second series, vol. XI, section IV, p. 48, 

 pi. II, fig. 5. 



i'h left upper premolar, worn. 

 Measurement in mm. 



p i a. p 12-5. 



A fourth left upper premolar is referable 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 ectoloph 

 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. brachystylus is from the Upper Oligocene, Leptauchenia beds of the 

 Cheyenne river, South Dakota, U.S.A. 



Mesohippus stenolophds, Lambe. 



Plate III, figs. 18, 19 and 20. 



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

 Assiniboia; Trans. Royal Soc. of Canada, second series, vol. XI, section IV, p. 48, pi. 

 II, tigs. 6, Ga and 66. 



1st left upper molar, unworn. 

 3rd right upper molar, unworn, (figures 18, 

 19 and 20). Type. 

 Measurements in mm. 



m x a. p 14. 



m 3 a. p 12-5 by tr 15. 



This species of Mesohippus is larger than M. brachystylus, but resembles it in some 

 particulars of its dentition. The differences are: (1) The greater relative size of m 3 , with a 

 more pronounced obliquity of the cross crests in these teeth, (2) the greater length of the 

 metaloph, which in m 3 is connected with the ectoloph, and (3) the intimate connexion of the 

 hvpostyle 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 pro- 

 toconule and a like suppression of the metaconule, (2) the parastyle and internal cingulum 

 similarly developed. 



A special character of M. stenolophus, seen in m 3 , is the oblique crossing of the parastyle 

 by the external cingulum, which rises rapidly from without and appears very distinct on the 



