TRANSACTIONS OF WAGNER 

 102 



UINTA SELENODONTS 



eminently so. They have the same low, broad crowns, with very concave ex- 

 ternal crescents, as in the White River type, though they betray their more 

 primitive character by the presence of the anterior intermediate cusp (proto- 

 conule), as in Protoreodon, and even in some specimens of Agriochcerus a rem- 

 nant of this unpaired lobe may be observed in unworn crowns of m^, where it 

 is represented by a slight elevation on the anterior horn of the antero-internal 

 crescent. In size the molars increase progressively from fn 1 to m-^, which 

 is the largest of the series. Between the molars of Protagriochcerus and those 

 of Protoreodon there is considerable resemblance, but the differences are ap- 

 parent at the first glance. In the former the external crescents have much 

 more deeply concave outer faces, while the median ribs upon these faces are 

 much less conspicuous ; the antero-external buttress is larger, heavier, more 

 rounded, and less compressed, while the median external buttress is penetrated 

 much farther by the median valley, which thus separates the external crescents 

 more widely ; the postero-external buttress is absent except on m^, where it 

 is small. The internal crescents are much alike in the two genera, though 

 in Protagriochcerus their apices are somewhat higher and the unpaired cusp 

 (protoconule) is somewhat more reduced. In both genera the internal cres- 

 cents are unequally developed, the front horn of the hinder crescent reaching 

 to the outer wall of the crown and cutting short the hinder horn of the 

 anterior crescent. 



As already mentioned, the resemblance of these molars to those of Agrio- 

 cluvrus is obvious at the first glance, and yet the latter shows a number of 

 changes from the Uinta type in molar structure. We observe (i) that the 

 protoconule has disappeared, or rather has become incorporated into the 

 antero-internal crescent; (2) that the median external buttress has become 

 more rounded and broader and is more deeply invaded by the median valley ; 

 (3) that the external crescents have become more deeply concave and their 

 median ribs have been suppressed, as has also the postero-external buttress of 

 m-^. In brief, the dentition of Agriochcerus differs from that of its presumable 

 predecessor, (1) in the presence of a diastema behind the canine, (2) in the 

 greater thickness of the premolars and the more complex patterns of p 4 and 

 p ? , and (3) in the slight changes of molar structure which have just been 

 enumerated. The differences between Protoreodon and Protagriochcerus are 

 sufficiently clear to the eye and yet they are rather difficult to express in 

 words. For this purpose the figures are more satisfactory than any descrip- 

 tion. 



