200 



Dakota and Nebraska. In Merycochoerus, on the other hand, before the 

 crown oft lie last molar was fully protruded, already the anatomical characters 

 of the triturating surfaces of those in advance were, to some extent, destroyed ; 

 and in the case of the first true molar completely obliterated, in this state pre- 

 senting simply a broad dentinal surface bordered with enamel. This condi- 

 tion is represented in the Fig. 3, of Plate X, of the same work just mentioned, 

 though in this case the specimen belonged to an individual past maturity, and 

 the last molar is .fully protruded. 



Another distinctive character in the teeth of Oreodon and Merycochoerus is 

 expressed in the less degree of transverse symmetry of the crowns of the 

 premolars in the latter. In Oreodon their various measurements are more 

 uniform, and the summits of the principal constituent lobes of their crowns are 

 nearly or quite median, and they nearly retained this relative position as the 

 teeth were worn away. In Merycochoerus the length and fore and aft diame- 

 ter of the crown exceed, the transverse diameter except in the last upper one; 

 and the summits of the lobes of the premolars, especially in the upper ones, 

 are more or less in advance of the middle of the crowns, and they likewise 

 retained this relative position as the teeth were worn. 



In the original description of Merycochoerus, its distinction from Oreodon 

 was mainly founded, on peculiarities of the skull; the differences in the teeth 

 above noted, especially those in the proportionate length of the crowns of the 

 molars, and their relative mode of protrusion, were not recognized. This 

 want of appreciation of the distinctive characters of the teeth of the two gen- 

 era arose from the observations having been made on the jaw-specimens of 

 Merycochoerus advanced beyond maturity, in which all the teeth were fully 

 protruded, and in this condition did not strikingly differ from those of Oreodon. 



A number of other fossils, discovered by Professor Hayden in the Pliocene 

 sands of the Niobrara Valley, and described by the writer at the same time 

 as those referred to Merycochoerus, from the difference in the proportionate 

 length of the molar teeth in comparison with those of Oreodon, were referred 

 to another genus with the name of Merychyus. In the same manner this 

 was supposed to differ from the Merycochoerus ; and though subsequently, in 

 the preparation of the " Extinct Mammalian Fauna of Dakota and Nebraska," 

 it was suspected that these two genera might prove to be the same, it was 

 not until the present moment the suspicion appeared to be confirmed. From 

 present observations and reflection, I am under the impression that Oreodon 



