PETERSON: THE AMERICAN DICERATHERES. 405 



Another feature, which often has been misinterpreted in connection with the 

 study of the Diceratheres, is the diiTerence in the contour of the slcuUs. It is a well- 

 known fact that in individuals of almost any group of mammals the contour of the 

 skull changes until well after complete maturity. Furthermore the sexual differ- 

 ences in the form of the skull in the Diceratherine branch of the Rhinocerotidse are 

 surprising. In the early development of the phylum the difference between the 

 sexes was well indicated by the form of the skull. ^ This is undoubtedly due in 

 great measure to the possession of the prominent nasal horns by the male. In a 

 young male, for instance of D. cooki, there are found the incipient horn-cores, the 

 nasals are quite long and pointed in front of the horns, while back of the horns 

 there is relatively small lateral constriction of the nasals, the temporal ridges are 

 generally weak and not united to form a sagittal crest, the zygomatic arches are 

 slender, often without, or with very slight, rugosities on the posterior angles. 

 This is also quite generally true of the skull of an adult female, with the exception 

 that in the latter there is a gradation from skulls without any horn-cores in the 

 young, to those having incipient horn-cores in some of the fully adult and old, 

 and that there is considerable variation in the prominence of the temporal ridges 

 and the manner of their convergence before they reach the inion. I have as yet 

 never seen a well-developed and heavy horn-core, the ends of the nasals short and 

 blunt, the skull much constricted laterally back of the horn-cores, saddle-shaped on 

 top, with a sudden lateral expansion and heavy rugosities on the posterior angles 

 , of the zygomatic arches in D. cooki, except in association with well-worn or very 

 old dentitions. It is very plain to me that more latitude should be assigned to the 

 significance of the contour of the skull in the genus Diceratheriurn than has some- 

 times been done. In study and comparison especial pains should be taken (1) 

 to ascertain whether the skull is that of a male or a female, or of the young, or 

 not entirely adult animal ; (2) skulls of fully adult or old males are more uniform 

 in contour than any others, and therefore more reliable in establishing species; 

 (3) the significance of the crushing received by the specimen in one direction or the 

 other should be noted. 



The following table of comparisons represents fairly well the large number 

 of skulls of Diceratheriurn cooki in the collection of the Carnegie Museum. Remains 

 of very young animals are not included in this table, but will be treated separately. 

 The object of the descriptions given under H, Nos. 2816, 2463, and 2478 in the 

 following table are to draw attention to the great ease by which misinterpreta- 

 tions may result with only a portion of the skull in hand and displaying charac- 

 ters, some of which may be only pathological. 



' Osborn, Henry F., "The Extinct Rhinoceroses," Mem. Am. Mus. Nat. Hist., Vol. I, Part III, 

 1898, p. 162. 



