456 MEMOIRS OF THE CARNEGIE MUSEUM. 



the majoritj' of cases is empty, the small canine lodged therein having dropped 

 out, while back of the canine we find a long diastema which is very much constricted 

 forming externally a deep and rather broad groove for the lodgment of the inferior 

 labial muscles. 



Let us suppose that the foetal specimens referred to have the jaws more or 

 less like those of the early Tertiary forms. We must in any event expect that the 

 early progenitors of the family had, first a complete dental series, i.e., f • {■ i- f 

 (abundantly proven by the genus Trigonias of the lower Oligocene); and secondly 

 quite likely the absence of a diastema back of the incisors.''^ It follows that 

 advancing influences effected gradual changes of the bony structure simultaneously 

 with that of the teeth. If we have, for instance, a set of lower incisors of subequal 

 size and a normal canine in its natural position (we actually do find evidence of a 

 canine in young Diceratheres), we should expect the upper incisors to meet the 

 lower. When the atrophied and hypertrophied changes took place, which trans- 

 formed the original sub-equal teeth to those which obtained in later forms, it was 

 not the lower canine, hut l-i, which received the constant impact from the upper median 

 tooth. The diastemata between the incisors, canine, and cheek-teeth was most 

 likely an early development of the group. The modification of the cutting incisor 

 was cotemporaneous with the reduction of Ii the atrophy of I3, the broadening of 

 the chin, and the constriction of the ramus in the region of the canine which, in 

 turn probably, caused the reduction and final disappearance of the latter tooth. 

 After the present study of the Diceratherinw, I cannot accept the designation 

 given to this tooth, as "canine," by some authors. Professors Marsh, Cope, and 

 Gaudry having been the first to promulgate this view. 



Since the preceding jmragraphs were submitted for publication, Professor 

 W. B. Scott of Princeton University has published his splendid work on the "His- 

 tory of Land Mammals in the Western Hemisphere." On consulting the history 

 of the Rhinocerotidse in Scott's volume, pp. 326-340, it is very evident that he 

 does not regard the large cutting incisor of the lower jaw in the early Rhinoceroses 

 as a canine. In fact since the genus Trigonias from the Lower Oligocene of America 

 was established by Mr. Lucas '^ and more completely described by Mr. Hatcher'^ 

 the morphology of the incisors and canines of the Rhinocerotidse rests on a firmer 

 foundation. 



'* Even in Colonoccran agrestis Marsh, a genus which might be regarded as possibly near the ancestral 

 line of the Diceratheres, there is a well-established diastema back of the superior canines. 

 "Proc. National Museum, Vol. XXIII, 1900, pp. 221-223. 

 'I' Ann. Carnegie Museum, Vol. I, 1901, pp. 135-144. 



