426 MEMOIRS OF THE CARNEGIE MUSEUM. 



material it appears that there is some variation in this respect, judging from the 

 statements of Dr. Loomis. The statement made by the latter author that the 

 nasals project considerably beyond the horn-cores is characteristic of this species, 

 while in D. cooki the points of the nasals are much abbreviated in fully adult or 

 old males. 



The muzzle of the skull in D. niobrarense is apparently not shorter than in the 

 John Day forms, while the constrictions back of the horn-cores and in front of 

 the orbits are longer and gentler, due to the relative narrowness of the nasals 

 across the horn-cores and the narrower frontals. The location of the infra-orbital 

 foramen is similar to that in D. armatum, located well back from the border of 



Fig. 13. Dicer alherium niobrarense Peterson, No. 1022, Coll. Amherst Museum. X i- After an 



outline drawing by Dr. F. B. Loomis. 



the anterior nares due to the slight backward excavation of the latter. The 

 occiput is, however, not overhanging, as in the John Day forms, while the external 

 ear is sometimes enclosed below. 



As has been shown by Loomis and Peterson the molar-premolar dentition of 

 D. niobrarense is more primitive than in D. cooki, and more nearly like that of 

 D. armatum. In the latter species, which is clearly an older and rather primitive 

 type, we find that P^ is somewhat larger, the ectoloph of the grinders is thinner, 

 the different valleys wider, and the cingulum perhaps somewhat more prominent. 

 To judge from the scanty remains of D. armatum which we possess, it certainly 

 is indicated that the crista is practically wanting, while the crochet is in a very 

 much more rudimentary stage of development on the teeth of the latter species 

 than in D. niobrarense. 



In comparing the descriptions and figures of Aceratherium egrerius, later 

 called Metacoenopus (I.e.) by Mr. Harold J. Cook, it is very evident that the remains 

 of an adult female of Diceratherium niobrarense has been used as the type. Cook 

 admits that there is a "thickening of the nasals at the point where a horn visually 

 occurs in the Rhinocerotidse, which may indicate a rudimentary horn." Indeed 

 one should expect to find this thickened condition, and we usually do find it in 



