PETERSON: THE AMERICAN DICERATHERES. 



433 



species, D. loomisi, is also established on a maxilla of D. cooki with deciduous 

 teeth. (Neb. Geol. Surv., VII, 1912, p. 29.) 



In comparing the crania of the abundant material of D. cooki with the earlier 

 John Day forms, or even with D. niobrarense fovmd in the same beds in which 

 D. cooki is found in Nebraska, it is at once clear that D. cooki is a comparatively 

 more specialized and modified type of the Diceratherina'. We find in the male 

 skull (1) a pair of prominent horn-cores set closely together on the nasals; the 

 nasals themselves not nearly as heavy as in the earlier John Day forms; and the 

 ends of the nasals much abbreviated as in more specialized or modified forms of 

 the TitanotheridcB] (2) muzzle, premaxillaries, and the front of the lower jaws 

 shortened and the lateral margin of the anterior nares extended further back of 

 the horn-cores; (3) brain-case enlarged; occiput broadening and not overhanging; 

 basicranium short, analogous to Teleoceras from the middle Miocene and the recent 

 Rhinoceroses {R. bicornis) ; (4) zygomatic arches much expanded with heavy rugo- 

 sities on the posterior angle, and the angle of the lower jaws heavy and very much 



Fig. 16. Dicerathcrium cooki Peterson. No. 1853, Coll. Carnegie Mu.seum. Skull of an old female. X i- 



everted to support heavy masseteric muscles ; (5) the grinding surface of the cheek- 

 teeth further complicated with a tendency on the part of the crochet to become 

 united with the ectoloph, especially in teeth having received some wear. 



As has already been stated in the introductory paragraphs of this paper, a 

 greater range of variability must be allowed in dealing with female crania of this 

 species. They range from those without horns in very young and immature 

 females to those with incipient horns in fully adult and old individuals. The 

 top of the skull is consequently comparatively little concave antero-posteriorly 

 and the supratemporal ridges vary so much in their course to the inion that this 

 region of the cranium may be said to range from a broad surface to a completely 

 formed sagittal crest (this variation of the supratemporal and sagittal ridges 

 holds good even in males, though to a somewhat less extent). Another feature 

 of the female skull, no less noticeable, is seen in the longer pointed nasals, the 



