MESOHIPPUS. 



39 



Type— Ottawa Mus. 6289, a fractured right upper molar and two doubtfully associated right lower molars (Ottawa 

 Mus. 6313). Measurements: nr a.p. .0095, tr. .012+, height of hypocone .004, ectoloph .005; associated lower molar 

 (paratype) a.p. .0095. The type upper molar described by Cope is imperfect, the outer slope of the ectoloph is missing, 

 and the anterior border of the crown, including the greater part of the protoloph, is very much damaged. Topotypc. 1 

 Ottawa Mus. 6293 (1905). Second superior molar of right side (Lambe, op. cit. Plate XIV, Figs. 1-4). Measurements: 

 tr. .013, a.p. .0102, height of ectoloph .0062. (Lambe, 1915, p. 244) The topotype described by Lambe is a perfect tooth, 

 in which the absence of the hypostyle indicates that this species is one of the most primitive of the known horses of 

 Oligoeene age. 



Type figure. — Text Fig. 18 of this Memoir. 



Characters. — (Osborn, 1904) Superior molar extremely primitive and brachyodont; transverse diameter propor- 

 tionately great; crests low and obtuse; persistent, well defined internal cingulum; protoloph with defined protoconule; 

 metaloph continuous, with metaconule somewhat less clearly defined; no hypostyle. 



Mesohippus latidens Douglass, 1903. 

 Text Fig. 19. 



Mesohippus latidens sp. nov., Douglass, Earl. "New Vertebrates from the Montana Tertiary," Ann. Carnegie Mus., Vol. 

 II, No. 2,1903, pp. 161-162, fig. 7. 



Hon zon and locality. — Lower White River beds, Thompson's Creek, near Three Forks, southwestern Montana. Type 

 collected by Earl Douglass in 1899. 



Fig. 19. Original figure of type of Mesohippus latidens Douglass, Carnegie Mus. 751, fourth upper premolar and three 

 molars, p 4 -m 3 , of the left side. Natural size. After Douglass, 1904, fig. 7, p. 161. 



Type. — Carnegie Mus. 751, four superior grinders, p 4 -m 3 , of the left side. Measurements: m 1-3 .032; m 1 a.p. .010, 

 tr. .016. 



Type figure. — ■ Text Fig. 19 of this Memoir. 



Characters. — (Douglass, 1903, p. 161) "The most striking distinguishing character of the teeth is the extreme 

 transverse as compared with the antero-posterior diameter and much greater width of the anterior than of the posterior 

 portions." (Osborn, 1904) Much more progressive than M. westoni; protoloph and metaloph more elevated; internal 

 cingulum not continuous; hypostyle wanting; distinguished from M. westoni and M. celer by prominent parastyle and 

 elongate protoloph so that anterior half of the crown is much broader than the posterior half. 



Mesohippus montanensis Osborn, 1904. 

 Plate 2.1. Text Fig. 20. 



Mesohippus montanensis, sp. nov., Osborn, Henry F. "New Oligoeene Horses," Bull. Amer. Mus. Nat. Hist., Vol. XX, Art. XIII, 

 May 28, 1904, p. 170, no figure. 



Horizon and locality. — Type and cotypes from Pipestone Creek beds, Jefferson County, Montana. Lower Titano- 

 therium zone. Type collected by W. D. Matthew of Am. Mus. Expedition of 1902. 



1 Lambe, Lawrence M. "On the Tooth-Structure of Mesohippus westoni (Cope)," Amer. Geologist, Vol. XXXV, Apr., 190o, 

 pp. 243-245, pi. XIV, figs. 1-4, 



