Troxell — Diceratherium and the Diceratheres. 207 



and the everted angle. A comparison with C. tridactylus 

 proavitus offers a further answer to the question of the 

 ancestry of M. coohi, for in both we see the broad sagittal 

 crest with converging straight lines, the heavy rugose 

 angles of the temporal bone, and in the former an early 

 stage of horn evolution. 



CcBfiopus tridactylus in the Oligocene shows foldings of 

 the enamel wliich may have given rise to the complex 

 pattern of M. coohi, and it is reasonable to suppose that, 

 just as favorable conditions must have influenced the 

 opulent development of D. armatum in Oregon, so an 

 unfavorable environment required a better dental mecha- 

 nism and greater protection by the horns, at the expense 

 of increased stature, on the Great Plains: M. coohi is 

 scarcely two thirds as large as D. armatum. 



Fig. 



Fig. 8. — Milk teeth, Dp^'.^, of Diceratherium annectens? Cat. No. 12003, 

 Y. P. M. For comparison with those of Menoceras cooM, see Peterson 1920, 

 pL 65, fig. 2. X 1/3. 



The anterior portion of the skull and jaws (No. 12500, 

 Y.P.M.) of a specimen of M. coohi was found by Pro- 

 fessor Lull in 1908 near Rawhide Buttes, Wyoming, show- 

 ing the distribution of the species in places other than 

 the Agate Spring quarries. 



Metaccenopus egregius (Cook). X 



M. egregius (Cook) has for its holotype a fine skull 

 and portions of the lower jaws in the private collection 

 of its author. Although the species is considered by 

 Peterson (1920) to be a synonym of M. niobrarensis, it 

 seems to warrant the rank of a subspecies at least, and in 

 any case should be retained as the type of the genus so 

 happily named. Important features of the genus are: 

 the larger size, smaller horn rugosities on the male 

 skulls, the simpler teeth, and a distribution limited to the 

 Great Plains. From the shape of the skull and the incipi- 



