THE MAMMALIA OF THE DEEP RIVER BEDS. 79 



I. Teeth brachyodont. 



A. (Jonules of upper cheek teeth well marked ; posterior transverse crest not reaching the outer wall ; external 



cusps moderately concave or flattened ; anterior pillar of lower teeth distinctly marked. 



1. No cement present. 



a. Incisors without enamel invagination *Mesohippus. 



h. Upper incisors with enamel pit Miohippus. 



2. Cement on cheek-teeth. 

 Posterior transverse crest of upper molars and premolars confluent with outer wall Desmaiippus. 



B. Conules of upper cheek-teeth much reduced and external cusps deeply concave ; posterior transverse crest 



extending to outer wall ; anterior pillar reduced and on one or more lower teeth absent ; no 

 cement Anchitherium. 



II. Teeth hypsodont. 



1. Antero-internal cusp of upper cheek- teeth confluent with anterior crest Protohippus. 



2. Antero-internal cusp separate from transverse crest Hipparion. 



MIOHIPPUS Marsh. 



American Journ. Science, Third Series, Vol. VII, p. 249. 



(Syn. Anchitherium Leidy, Cope and Marsh, in part.) 



This genus was proposed on the absence of the lachrymal fossa in the type 

 species, but Prof. Cope informs me that this character is not of more than specific 

 value, as in Protohippus and Hipparion, as well as in the John Day genus, some 

 species have it, while others lack it. As shown in the foregoing table, Miohippus is 

 sufficiently well distinguished from Anchitherium, but the propriety of its separation 

 from Mesohippus mast remain doubtful until the upper incisors of the latter genus 

 have been found. 



The John Day species have all been established on characters taken from the 

 upper molars, and, as there are no upper teeth comprised in the present collection, 

 the reference of the species can only be approximate. 



Miohippus equiceps? Cope. 



(Syn. Anchitherium equiceps Cope, Proceedings Philos. Soc, Vol. XVIII.) 



The lower beds of the Deep River (i. e., upper John Day) yielded some mandi- 

 bles which agree fairly well with the smaller individuals of this species, to which 

 they may be provisionally referred. 



* The upper incisors of this genus are not known, and future discovery may show that it is not generically 

 different from Miohippus, but the generally less advanced character of the dentition renders it probable that the 

 character of the incisors is as assumed above. 



A. P. S. — VOL. XVIII. K. 



