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PROCEEDINGS OF THE NA TIONAL MUSEUM. 



VOL. XXIII. 



three incisors and a canine, having therefore the most generalized den- 

 tition of any rhinoceros thus far discovered. The canine and second 

 and tliird incisors are of tlio same general shape, l)eing slightly com- 

 pressed with rounded points; the canine is the smallest tooth in the 

 series and the teeth progressively increase in size from the canines 

 forward. The three premolars are practically of the same size, as 

 those shown on Plate Xlll, lig. 7, of Os]>orn's memoir on the Extinct 

 Khinoceroses, but exhibit a greater degree of wear. The first pre- 

 molar is, however, slightly more elongate and less trihedral in sectioji 

 than the first premolar there shown, while the protoloph is narrower, 

 lies on the extreme inner edge of the tooth, and runs directly luickward. 

 The left ranuis of a jaw, inchiding the entire s^nnphysial portion, 

 also from the Miocene of South Dakota, is assigned to this species, as 



Fig. 2. — Symphysis of jaw of Tkigonias osborni. \ 



the two specimens arc from animals evidently of the same size, and the 

 peculiarities of dentition shown by the jaw are in accord with those of 

 the portion of cranium just described. The jaw is numbered 4815 in 

 the catalogue of fossil vertebrates, U. S. National Museum. The 

 important portion of the jaAV is the symphysis, which bears on either 

 side a small inner incisor, the root of which onh" is present, a large 

 procumbent tooth, usually regarded as a canine, and back of this an 

 alveolus for a third and small tooth. The presence of this tooth shows 

 conclusively that, whether it be regarded as a canine or an incisor, the 

 large procumbent tooth nuist be an incisor. Until proof to the con- 

 trary is forthcoming, I prefer to look upon the small tooth as the third 

 incisor and to regard the canine as absent. 



The lower grinders increase considera))ly in size from before back- 

 ward, so that while the second and tliird premolars are of nearly the 



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