1887.] -^57 [Scott and Osborn. 



A second species of rodent is represented by a lower incisor and part of 

 the mandible, but as no molars are preserved, the specimen cannot be 

 referred to any genus. The Incisor is very much larger than in any known 

 species of Plesiarctomys, and equals that of the beaver in size. It may 

 not improbably belong to some ancestral member of the Castoridm, as that 

 family is known to exist in the overlying White River beds. 



Measurements. 



M. 



Antero-po.sterior diameter of lower incisor 008 



Transverse " " " 0065 



Artiodactyla. 



Protoreodon parvus, gen. et sp. nov. (JAgrioclioeruslsl&r&Xx). — This genus 

 is of interest as being the direct forerunner of the Oreodontida, which are 

 so characteristic of the American Miocene formations. It differs from 

 Oreodon in the structure of the upper molars, which have five cusps, an 

 intermediate one appearing between the anterior pair. The crowns of the 

 upper molars are very wide and the internal crescents separated from the 

 external by a considerable interval, much as in Agriochoerus. Both upper 

 and lower premolars are somewhat simpler than in Oreodon, and the latter 

 are trenchant, as in Tragidus. It differs very radically from Agriochoerxis 

 in the character of the premolars as well as in the presence of the inter- 

 mediate cusp in the superior molars. The orbit is not enclosed behind, 

 the cranium is narrower and less rounded and the brain smaller than in 

 Oreodon. On the other hand, its relationship with that genus is incon- 

 testable ; the lower teeth form a closed series without any diastemata, the 

 lower canine has gone over to the incisor series, and the first premolar has 

 taken on the form and function of the canine, a peculiarity found only 

 among the Oreodontidai. As in that family, the mandible is short, deep, 

 and has an abruptly rounded chin. 



The number of digits is uncertain, but is probably v-iv, as is unques- 

 tionably the case in Oreodon ; the magnum has not moved so far to the 

 radical side, but is more directly under the magnum than in the latter 

 genus. In the tarsus the external and median cuneiforms are fused 

 together as in Oreodon, but the line of union is much more plainly marked 

 than in that genus. 



Protoreodon parvus : Upper molar series, natural size. Tlie inner cusp of the third 

 premolar has been broken away. 



This species was of very small size, much inferior to Oreodon gracilis. 



PKOC. AMER PHILOS. SOC. XXIV. 126. 2G. PRINTED NOV. 2, 1887. 



