36 



osborn: OLIGOCENE, MIOCENE, pliocene equid^e. 



GENUS MESOHIPPUS Marsh, 1875. 

 Amer. Jour. Sci. and Arts (3) Vol. IX, Mar. 1875, p. 248. 



Genotype.— Anchitherium (Paleeotherium) bairdii Leidy, from the Lower Oligocene, lower Oreodon beds of the White 

 River series of South Dakota, 



Nineteen species have been described under this stage of equine evolution between the years 1850 and 1908, besides 

 six which have been removed to Miohippus. 



The principal stages of character genesis and of proportional evolution comprised within the genus Mesohippus are as 

 follows: 



1. Premolars ^ of equal or less length than molars j|. 



2. Premolars |^ typically of equal or less width than molars jjjf. 



3. Facial (preorbital) less than cranial (orbito-postorbital) length. 



4. Molars typically with internal cingulum vestigial or wanting except in lower Titanotherium zone, external cingu- 

 Lum more or less clearly defined. 



5. Lophs more or less interrupted, i. c. bunolophodont, or crested, i. e. lophodont. 



6. Hypostyle absent, rudimentary, or strongly developed (M. trigonostylus). 



7. A rudimentary crochet in M. trigonostylus. 



8. iV shallow lachrymal fossa or a lachrymo-malar fossa (Plate 1). 



Comparison of the type and referred skulls (Plate 1) and dental series (Plate 2) proves conclusively that Mesohippus 

 embraces not only many stages of evolution during the period of the Titanotherium zone and of the Oreodon zone but 

 also a number of distinct and separate phyla, as follows: 



I. Titanotherium zone species. 



Teeth moderately crested 

 No hypostyle 



Lateral digits less reduced, Mtc. V splint apparently long 

 Mts. cuboid facet absent or very small 

 M 3 somewhat reduced 



II. Oreodon zone species, Mesohippus bairdii group. 

 Teeth moderately crested 

 Hypostyle distinct 



Upper premolars equalling or slightly smaller than molars 



Facial region short 



Shaft of digits slightly less elongate 



Mts. Ill cuboid barely in contact 



III. Metamynodon zone, M. trigonostylus group. 



Hypostyle very prominent, triangular 

 Lophs of bunolophodont type 



IV. Titanotherium and Oreodon zone, Mesohippus 



eulophus group. 



Hypostyle persistently rudimentary 



Lophs typically crested 



Lachrymal fossa; deep, superior in position 



TITANOTHERIUM-MESOHIPPUS ZONE. 1. LOWER OLIGOCENE. 



This life zone, geographically distributed in South Dakota, Colorado, Nebraska, Montana, and British 

 Columbia, as described above, contains the smallest and most primitive species of Mesohippus. 



Mesohippus celer Marsh, 1874. 

 Plate 2.9. Text Fig. 17. 



Anchitherium celer, Marsh, 0. C. "Notice of New Equine Mammals from the Tertiary Formation," Amer. Jour. Sci. and Arts 

 (3) Vol. VII, No. 39, Mar. 1874, p. 251, no figure. 



Horizon and locality. — Miocene of Nebraska (Marsh). Level unrecorded, probably Lower Oligocene, Titanotherium 

 zone. Type collected by W. A. Jones. 



Type. — Yale Mus. 11302, superior molars, p 4 -m 3 , in a portion of the right maxillary. Measurements: m 1-3 .027 

 (Marsh), .0295 (Gidley); m 1 a.p. .010, tr. .015; m 3 a.p. .0085, tr. .012. 



Type figure. — Plate 2.9, text Fig. 17 of this Memoir. 



