Troxell — Diceratherium and the Dicer atheres. 199 



ense) ; (3) cingula broken on the molars, weak on the 

 tetartocones, with a tendency toward elimination; (4) 

 development of minnte folds into a crochet, but virtual 

 absence of a true crista; (5) moderate grooves on the 

 protoloph of molars, separating the protocones from the 

 protoconules, absent on premolars; (6) incisors and 

 canines lost from premaxillary, except first and possibly 

 second incisor; (7) milk dentition rather complex; (8) 

 in size one of the largest rhinoceroses of the time; (9) 

 geological age, middle John Day, corresponding to the 

 Upper Oligocene or Lower Miocene of the Great Plains. 



Fig. 1. 



\ 



Fig. 1. — Premaxillaries showing the development of incisors in various 

 species of Diceratherium in the Yale collections. All shown from the left 

 outer side. X 1/3- 



A, D. armatum. Cat. No. 10005. Second incisor unknown and bone 

 restored in outline. 



B, D. armatum. Cat. No. 11068. Two moderately large incisors present. 



C, D. lohatum, sp. nov. Holotype. Cat. No. 12487. Lobate character 

 of incisors and relatively large size of second one are unusual features. See 

 figure 6 and the text description. 



D, D. nanum. Cat. No. 11184. Drawn reversed. Note persistent small 

 second incisor. In this old individual the prominent horn rugosities are 

 wide spreading. 



E, D. nanum. Holotype. Cat. No. 10004. See figure 2 and the text 

 description. No second incisor present. 



F, D. annectens. Holotype. Cat. No. 10001. See the text description 

 following, and also figure 3. 



G, D. annectens. Cat. No. 12019. A young specimen with premolars 

 almost exactly like those of the holotype (fig. 3) but slightly smaller. Nasals 

 broad but no prominences have developed. 



Diceratherium annectens (Marsh) is a smaller species 

 of rhinoceros, also from Oregon; it was named by Marsh 

 from a specimen (Cat. No. 10001, Y.P.M.) consisting 



