DAKOTA AND NEBRASKA. 121 



Merychyus major. 



The largest species of Merycliyas, distinguished by the above name, is indicated 

 by fragments of jaws with teetli, from several different individuals. The specimens 

 indicate an animal approximating in size Merycochcerus. 



An upper jaw fragment, containing the third and fourth premolars and the 

 succeeding two true molars, is represented in figures 15, 16, plate XI. The teeth 

 are surrounded by a well-marked, though thin basal ridge, which is feebly developed 

 only at a few points in the smallest species. In the second premolar a large pointed 

 tubercle assumes the position of the acute ridge, situated antero-internally in the 

 corresponding tooth of the smallest species and in that of Oreodon. 



A singular peculiarity in the teeth of this specimen is the apparent absence of 

 enamel in certain positions in which it ordinarily exists before it is removed by 

 attrition. Thus it appears to be absent on the external surfaces of the inner lobes of 

 the true molars. The enamel on the internal surfaces of the outer lobes thins away 

 and appears to cease at their lateral borders, or turns for a short distance over the 

 contiguous edges of the external surfaces of the inner lobes. The enamel also appears 

 absent on the external surface of the inner lobe of the last premolar, and for some 

 distance at the sides of the internal surface of the outer lobe. In the third premolar 

 the enamel appears absent on the external surflice of the postero-internal sub-lobe, 

 and the greater extent of surface bounding the interspace externally between the 

 sub-lobe and the principal lobe. Where the enamel appears to be absent, it is no 

 doubt present in a much thinned condition, so as not to be obvious under ordinary 

 inspection. 



The measurements of the specimen are as follow : 



Lines. 



Antero-posterior diameter of the second true molar, . . .16 



Transverse « « « .... 13} 



Antero-posterior diameter of first true molar, .... 12i 



Transverse " " " . . . . .11 



Antero-posterior diameter of fourth premolar, . . . . 8i 



Transverse " " " . . . . . 9i 



Antero-posterior diameter of third premolar, . . , .8} 



Transverse " an .... 71 



A fragment of a lower jaw of another individual, containing the last molar with the 

 crown half worn away, presents, as the measurements of the tooth, an antero-posterior 

 diameter of twenty-oue lines, and a transverse diameter of eight and three-quarter 

 lines. 



Another fragment, containing the true molars much worn and mutilated, proves 



IG 



