PETERSON : THE AMERICAN DICERATHERES. 427 



the .young males and in female skulls of D. cooki of more adult stages. It appears 

 that in males of D. niobrarense the nasal horn-cores are located more laterally and 

 point more outward,-' while in D. cooki they are nearer the median line and point 

 more directly upward. (See PL LXII.) 



With the exception of the longer nasals in front of the thickening portion or 

 the incipient horn-cores (undoubtedly a sexual character), Mr. Cook's description 

 agrees quite well with the type of D. niobrarense. 



The premaxillaries are complete in this splendidly preserved specimen in 

 Mr. Cook's collection, and show some reduction in length from those in the older 

 John Day forms. 



There is a considerable portion of the left mandible in Mr. Cook's specimen, 

 which was found in an articulated position on the skull. Cook states that this 

 mandible is "heavier and lacks the outward turn or flange commonly found in 

 the Diceratheres." A splendid pair of lower jaws in the Loomis collection at 

 Amherst (see Fig. 14) referred to D. niobrarense also agrees with the characteri- 



FiG. 14. Diceratherium niobrarense Peterson, No. 1022, Coll. Amherst Museum. Internal view of 

 ramus. X i- After an outline drawing by Dr. F. B. Loomis. 



zation in Cook's paper, and plainly indicates that the masseteric muscle was 

 much less developed in this species. Unfortunately the outline drawing kindly 

 furnished by Professor Loomis from the Amherst specimen does not indicate the 

 external view of the mandible. 



The atlas and axis of Mr. Cook's specimen were found attached to the occipital 

 condyle and are so illustrated in his paper. No description in detail of these 

 vertebrae is, however, furnished. 



A series of cervicals (atlas, 3d, 4th, and 6th), an anterior dorsal (?5) and three 

 lumbar vertebrae were found isolated in the same quarry (quarry A) in which 

 the type of D. niobrarense and the Amherst material was found. These bones, 

 No. 1910, are here provisionally referred to D. niobrarense, inasmuch as the size 



''^ Figures on Plate I of Cook's illustrations show admirably well these lateral eminences although of 

 an incipient stage most likely due to sex. Cook states that there is no double-horn tendency in his type. 



