MESOHIPPUS. 



41 



style, (6) metaloph narrow and nearly connected with ectoloph, (7) protoconule easily distinguishable, but (8) metaconule 

 absent, (9) a crotchet present on the metaloph, and (10) a small conule in the posterior valley of the tooth, (11) a rudiment 

 of a cingulum between protocone and hypocone, (12) parastyle and (13) hypostyle small." 



(Osborn, 1918) The exceptionally progressive characters of these type molar teeth, including the purely lophoid 

 metaloph and the rudimentary crochet, indicate that this animal may belong to a higher geologic horizon than the Titano- 

 therium zone. 



Mesohippus hypostylus Osborn, 1904. 



Plates 1.2, 2.3. Text Fig. 22. 



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

 May 28, 1904, pp. 170, 171, fig. 2, pi. V A. 



Horizon and Locality.— Cheyenne River, South Dakota, Upper Titanotherium zone. Type collected by the Am. 

 Mus. Expedition of 1894. 



Type. — Amer. Mus. 11S0, preorbital portion of skull, palate with complete series of grinding teeth on both sides, 

 p'-m 3 . Measurements: p'-m 3 .075; m 1-3 .032; m 1 a.p. .010, tr. .0135; vertical hypocone .006; ectoloph .0075. 

 Type figure.— Text Fig. 22 of this Memoir. 



Characters. — (Osborn, 1904, p. 170) "Distinguished from the preceding species by (1) a clearly defined but rudi- 



Fig. 22. Original figures of the type of Mesohippus hypostylus Osborn, Amer. Mus. 1 ISO, which consists of preorbital 

 portion of skull and palate with complete series of grinding teeth on both sides, p L ra 3 . Skull one half natural size, teeth 

 natural size. 



mentary hypostyle just budding off from the posterior cingulum, (2) by protoloph tending to unite with paracone [para- 

 style]; from M. bairdi[i] by more rudimentary hypostyle and less elevated crests. Metaloph sharp; metaconule not 

 defined at base. Pm 1 small. Skull with preorbital [lachrymal] fossa apparently deeper than in M. bairdi[i\." 



Mesohippus proteulophus Osborn, 1904. 



Plates 1.1, 2.2. Text Fig. 23. 



Mesohippus proteulophus, sp. nov., Osborn, Henry F. "New Oligocene Horses," Bull. Amer. Mus. Nat. Hist., Vol. XX, Art. 

 XIII, May 28, 1904, pp. 171-172, no figure. 



Horizon and locality. — Badlands near Cheyenne River, South Dakota, Upper Titanotherium zone. Type collected 

 by Am. Mus. Expedition of 1892. 



Type. — Amer. Mus. 514a, upper grinding teeth, p 4 -m 2 . Cotypc. Amer. Mus. 1208, a lower jaw. 

 Type figure. — Plates 1.1, 2.2, text Fig. 23 of this Memoir. 



Characters. — (Osborn, 1904) "This is a relatively large animal for the Titanotherium beds, and is the oldest horse 

 known with perfect crests on the molar teeth — hence the name proteulophus. The superior molars are readily distinguished 

 by: (1) their large size; p 3 -m 2 measure .037 mm. as compared with .0335 for the same teeth in M. bairdi[i]; (2) by 

 the primitive absence of a distinct hypostyle; (3) especially by the very advanced or progressive condition of the proto- 



