VERTEBRATE FAUNA OF THE LOUP FORK BEDS. 41 



The fossils include metapodial and the tarsal bones of a rhinoceros pre- 

 sumably of the genus Aphelops; fragments of a superior and inferior 

 molars of a true horse (Equus) ; and superior molars of horses probably 

 of the genera Hippidium and Protohippus. Neither the first nor last 

 named of these genera has been found in the Blanco, while they do occur 

 in the Loup Fork. The genus Equus, however, is not known in the lat- 

 ter formation, but first appears in the Blanco. The fragment preserved 

 is quite characteristic, and I see no way at present of distinguishing it 

 from the Equus simplicidens, Cope, of the Blanco bed, but better speci- 

 mens may indicate another identification. The siiecies which I can iden- 

 tify are the following: 



PROTOHIPPUS LENTICULARIS, CopC. 



Sp. nov. Plate XII, Figures 1, 2. 



Two superior molar teeth represent this horse in the collection from 

 Goodnight' s. They indicate a species of the size of the Hippotherium 

 speciosum, and present several peculiarities. The most conspicuous is 

 seen in the form of the protocone, which is long separate from the pai'a- 

 conule. It has an absolutely lenticular section, presenting acute angles 

 anteriorlj'^ and posteriorly, and convex surface internally and externally. 

 In this respect it differs from all the species of Hippotherium and Proto- 

 hippus known to me. The column is fused towards the base with the 

 paraconule, so that an old animal belongs to the genus Protohippus, as in 

 the case with the Protohippus placidus, described on a preceding page. 

 As both the teeth of P. lenticularis are only partially worn, the pattern of 

 their grinding faces is that of the genus Hippotherium. 



The enamel borders of the lakes are moderately complex, the anterior 

 having the posterior loop nearly isolated, and a trace only of other irreg- 

 ularities, while there are two deep notches of adjacent borders of the pos- 

 terior lake. The remote borders of the lakes have each a mere trace of a 

 notch. There is one deep loop of the internal border, which extends to 

 the protocone in one tooth and nearly to it in the other. It is rarely so 

 pronounced in the three-toed horses, and when large is frequentl}^ double. 



The crown is curved transversely but not anteroposteriorly. 



Measureme7its . 



Mm. 



i longitudinal 48 

 transverse 19 

 anteroposterior 21 



Diameters crown lio. 2 j longitudinal 44 



transverse 18 



I 



The long separation of the protocone and paraconule allies this species 

 to the P. placidus, Leidy, but from the latter it is well distinguished by 

 its superior size and by the shape of the protocone. The Hippotherium 



