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ami Merycochoerus are two quite distinct though closely allied genera, of 

 which the latter is geologically the later, and, perhaps, the successor by evolu- 

 tion from the former. Merychyus would appear to be the same as Meryco- 

 chojrus, and the fossils which had been referred to it belong to the same geo- 

 logical horizon. 



Of the specimens originally attributed to Merychyus major and M. medius, 

 too little of the corresponding parts were preserved in such a condition as to 

 enable us to make a comparison of the upjier jaw and face with the same 

 parts in Merycochoerus to ascertain how far they are like one another. The 

 position of the infra-orbital foramen, which appears to be nearly or quite con- 

 stant in a species, or varies but slightly in several species of a genus, in the 

 jaw-specimen referred to Merychyus major is placed above the last premolar. 

 It occupies the same position in Merychyus elegans ; and in the up[)er jaw of 

 a young animal, referred to Merychyus Jiiedlus, it is placed aljove the last 

 temporary premolar, therefore agreeing in position with that in tlie adults of 

 the other two species. In Merycochcej-us proprius the position of the foramen 

 is further back, al)ove the interval of the first and second molars, and this is, 

 also, its position in the upper jaw of the Sweetwater species named Meryco- 

 choerus rusticus. 



This difference of position is probably related with adiiference in the shape 

 of the face, which in ]\Ierycochoerus is rather abruptly narrowed in advance 

 of the zygomata, as in the hog. The face of Merychyus I suspect rather to 

 1)0 more like that of Oreodon, nari'owing graduallj' forward from the position 

 of the orijits and zygomata, as in the peccary. 



Admitting the three genera, Oreodon, Merycochoerus, and Merychyus, their 

 distinctive characters, so far as ascertained from the materials at command, 

 would appeal' to be as follows : 



Oreodon. — Molar teeth with short crowns, as in the deer; and, as in 

 this, at maturity inserted by fangs. Anterior premolars straight, with the 

 diameters nearly equal, and with their points median or nearly so. Face 

 gradually convergent, conical. Infra-orbital arch narrow or of moderate 

 depth; gradually declining upon the side of the face. Infra-orbital foramen 

 small and situated al)ove the tliird premolar. Nasal orifice nearly as wide as 

 high, and situated immediately above the incisive alveolar border, as usual in 

 most animals. Premaxillaries and maxillaries remaining distinct from one 

 anotlier. Incisive foramina of moderate size. 

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