44 



osborn: OLIGOCENE, MIOCENE, pliocene eqtjid.e. 



larly developed. A special character of ill. stenolophus, seen in m 3 , is the oblique crossing of the parastyle by the external 

 cingulum which rises rapidly from without and appears very distinctly on the upper anterior surface of the style (Figs. 

 6a, 6b). The cross crests are narrow in proportion to their height, a feature suggested in the name given to the species." 



Lambe regards M. stenolophus (op. tit., p. 51) as approaching closely Midhippus brachystylus Osborn of the Leptau- 

 chenia zone. 



Mesohippus planidens Lambe, 1905. 

 Text Fig. 24(21). 



Mesohippus planidens, sp. nov., Lambe, Lawrence M. "Fossil Horses of the Oligocene of the Cypress Hills, Assiniboia," Trans. 

 Roy. Soc. Canada, 2nd Ser., Vol. XI, Sec. IV, 1905-1906, issued Aug., 1905, p. 49, PI. ii, fig. 7. 



Horizon and locality. — Oligocene deposits capping the Cypress Hills, Assiniboia, Titanotherium zone to Leptauchenia 

 zone. This type is regarded as of Protoceras zone age. Type collected by Lawrence M. Lambe. 



Type. — Ottawa Mus. 6304, first and second left upper molars, worn. Measurements: m 1-2 .031; m 1 a.p. .015, tr. 

 .0178. 



Type figure. — Text Fig. 24 of this Memoir. 



Characters. — (Lambe, 1905, p. 49) "These teeth indicate a species of about the size of M. intermedins, Osborn and 

 Wortman, from the Upper Oligocene, Protoceras beds of South Dakota, but smaller than M. valid us, 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 M. 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 hypocone are not so elevated as the ectoloph. 

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



Lambe observes {op. tit. p. 51) that M. planidens approaches in size Miohippus intermedins Osborn and Wortman 

 from the Protoceras zone. 



Mesohippus assiniboiensis Lambe, 1905. 

 Text Fig. 24(22,23,24). 



Mesohippus assiniboiensis, sp. nov., Lambe, Lawrence M. "Fossil Horses of the Oligocene of the Cypress Hills, Assiniboia," 

 Trans. Roy. Soc. Canada, 2nd Ser., Vol. XI, Sec. IV, 1905-1906, issued Aug., 1905, p. 50, PI. ii, figs. 8, 8a, 8b. 



Horizon and Locality. — Oligocene deposits capping the Cypress Hills, Assiniboia, Titanotherium zone to Leptauchenia 

 zone. This type is recorded as of Protoceras zone age. Type collected by Lawrence M. Lambe. 



Type. — Ottawa Mus. 6305, second right upper premolar, p 2 , unworn. Measurements: p 2 a.p. .015, tr. .017; 

 height or tritocone .0105; height of tetartocone .00S. 



Type figure. — Text Fig. 24 of this Memoir. 



Characters. — (Lambe, 1905). (1) The type of larger size than that of Miohippus intermedins or M. validus from the 

 Protoceras zone. (2) Greater development of parastyle than in M. brachystylus; (3) proto- and metalophs short, set at 

 right angles to ectoloph; (4) deutero- and tetartocones of premolars strongly and equally developed; (5) protoconule 

 much smaller than metaconule; (6) ectoloph with distinct mesostyle; (7) hypostyle tends to separate from posterior 

 cingulum; (S) cingulum robust, high and sharp. Lambe observes (op. tit. p. 50) that this animal, known only from the 

 second upper premolar, approaches most nearly in tooth development the much smaller Miohippus brachystylus Osborn. 

 It most closely resembles Miohippus vedidus in size, but is considered even larger. 



