1889.] ^^* [Cope. 



Tliis three-toed horse was of about the size of the Uippotherium sjiecio- 

 sum, which I have already shown to have been equal to the prong-horned 

 antelope, but of somewhat diiferent proportions. 



This species stands in a certain relation to the FrotoJdppus inslgnis 

 Leidy, a species which is common in the Eastern beds of Loup Fork age. 

 Should the anterior internal column unite with the anterior internal cres- 

 cent, thus developing the character of the genus Protohippus, it is doubt- 

 ful whether the two species would be distinguishable by the dentition. I 

 have not seen, however, any intermediate specimens, and the two species 

 occupy diflferent geographical areas. A somewhat similar relation exists 

 between the H. sphenodus and the P. perditus, but in the former the 

 enamel lake borders are much more complex. 



HiPPOTHERIUM SEVERSUM Cope. 



Stylonus seversus Cope, Paleontolog. Bulletin, No. 30, p. 14. Proceed. 

 Amer. Philos. Society, 1878, p. 76. 



The H. seversum was originally described from a superior molar, which 

 though worn over the entire grinding face of the crown still retains the 

 posterior column distinct from the posterior inner crescent. On this ac- 

 count I distinguished the species generically from Hippotherium under 

 the generic name Stylbnus. I think, however, that a little further wear 

 would reveal the union between the two parts of the crown above men- 

 tioned, which would then present characters not distinguishable from 

 those ot Hippotherium. 



Crown of superior molar moderately elongate and curved. Grinding 

 face subquadrate. External ridges prominent. Section of anterior internal 

 column oval, with an apex directed outwards at a short distance anterior 

 to the single internal median loop. Posterior internal column also oval 

 in section, distinct from posterior internal crescent, and without apiculate 

 angle in specimen at present state of wear. Crescents narrow, separated by 

 rather wide lakes entirely filled with cementum. Lake borders simple ; 

 anterior with a trace only of anterior notch ; posterior with a strong pos- 

 terior notch. Opposed adjacent borders with one notch on the posterior 

 and two on the anterior crescents, the latter enclosing a small loop in the 

 usual position. External cement layer rather thin. 



3Iea8U7'emenis. 



MM. 



r anteroposterior 18 



Diameters crown } transverse 17 



(^ longitudinal 28 



Ticholeptus bed of Cottonwood creek, Oregon ; Dr. J. L. Wortman. 



This species diflfers from both the S. isonesum and H. calamarium in 

 its inferior dimensions, and in the simplicity of its enamel lake borders. 



PROC. AMER. PHILOS. SOC. XXVI. 130. 3f. PRINTED SEPT. 11, 1889. 



