34 



with the parastyle, with which the outer end of the protoloph shows a marked tendency to 

 unite. Outwardly the cingulum rises on to the parastyle, but does not cross it. 



M. westoni, judging of its dental characters principally from the molar obtained in 1904, 

 approaches closely to M. latidens,* Douglass, in tooth-structure, but its molars are distin- 

 guished principally by the presence of an internal cingulum, by the less pronounced parastyle, 

 and a proportionately greater antero-posterior diameter, with the protoloph more nearly equal 

 in length to the metaloph, as well as by other characters. M. celer, ** Marsh, and M. mon- 

 tanensis, ^** Osborn, are two other nearly related, but apparently distinct species, from the 

 Lower Ohgocene. The presence of the highly developed internal cingulum is one of the 

 most interesting characters in the dentition of M. ivestoni. This character, together with the 

 absence of a hypostyle, points to this species being probably the most primitive of the known 

 horses of Oligocene age. 



A lower second premolar and a lower molar, are included in the collection of 1904. 

 Figures of the two lower molars obtained by Mr. Weston are given in Cope's memoir of 1891. 



Measurements in mm. Measurements Measurements 



of upper molar of M. of type molar of Mi (type) of 



H-es^oni obtained oiM.wcstonia& M. montanen- 



in 1904. given by sis as given 



Osborn. by Osborn. 



Transverse diameter , 13'0 12+ 14 



Antero-posterior diameter 10-2 95 



Height of protocone 4'5 10-5. 



.Height of hypocone 4 ■ 5 4- 



Height of ectoloph 6'2 5 + 



Mr. Gidley has expressed the opinion that, the tooth collected in 1904, approaches more 

 closely to M. montanensis than to M. westoni, and would be inclined to refer it to the former 

 species with some degree ot doubt. In comparing it with M. montanensis he has noted the 

 somewhat less elevated inner cones and the presence of an internal cingulum, regarded by 

 him as differences of perhaps not more than varietal value, which may be explained by calling 

 the tooth a premolar instead of a molar. 



Judging from the characters of the teeth and their size, it is evident that the two species 

 M. westoni and M. montanensis approach each other closely. The type of M. westoni is not 

 as well preserved (nor as accurately figured in the original description) as is desirable, and the 

 absence of the ectoloph and the anterior margin, including the front slope of the protoloph, 

 precludes the taking of exact measurements. That the tooth of 1904 (figures 10-13) comes 

 from the same locality as the type of M. westoni, and that it has a particularly well defined 

 internal cingulum, are facts worthy of every consideration in determining its specific affinity. 



Mbsohippus PRiBcociDENS, Lambe. 



Plate III, fig. 14. 



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

 Assiniboia; Trans. Royal Soc. of Canada, second series, vol. XL section IV t) 46 

 pi. II, fig. 2. , , , p. , 



1903. 



New Vertebrates from the Montana territory by Earl Douglass ; Annals of the Carnegie Museum, vol. II, p. 161, 



** 1874. American Journal of Science, vol. VII, p. 2,51 

 **» 1904. Op. cit. 



