PETERSON: THE AMERICAN DICERATHERES. 455 



7. In No. 1854 it is seen that the lateral incisor is no further advanced than in 

 No. 1841 just described. The alveole or deep groove^^ for the deciduous canine is 

 still quite prominent while D.Pi is shed and all traces of its alveole entirely oblit- 

 erated. P2 and P3 have received slightly more wear than those teeth in the previous 

 specimen described, but D.P4 is still well rooted in the alveolar border. Mi is quite 

 well worn and the anterior part of M2 is also more worn than that in No. 1841. 

 The deep fissure back of Mo indicates the position of M3. The latter is very little 

 further developed than in the two preceding specimens and is yet buried in the 

 angle of the jaw. The three lower jaws just described are of approximately the" 

 same age as the skull No. 1848, referred to in the opening paragraph of this dis- 

 cussion. 



In connection with the probable manner in which the upper and lower incisors 

 of Diceratheriuni^^ developed in size, and modified into the shape in which we find 

 them, it is interesting to return to the foetal specimens Nos. 1820 and 1821 just 

 described (page 452). We have already found that the deciduous dentition of 

 these specimens forms practically a close series, without the constricted and thin 

 areas of the alveolar border between the cheek-teeth and the incisors of older ani- 

 mals, the alveolus for the canine is deep though small; in excavating the chin, 

 the continuation of the dental canal is found, but the germ for the permanent 

 incisor had not yet started, hence the small transverse diameter of the chin. 



In the next stage represented by Nos. 2476 and 2477, the specimens are of 

 quite young animals. We observe here a sudden change. It is likely that the 

 characters so prominently developed in this young animal had already been 

 well advanced during the latter part of the intra-uterine stage. At all events 

 the jaw was still in a very plastic condition in order to have transformed so quickly 

 between the two stages represented in the illustrations (see PL LXVI, Figs. 7 and 

 9). In the specimens of this second stage we find a broad and heavy chin in order 

 to support the heavy and long-rooted permanent incisor. The alveolus for the 

 canine, which we originally found quite deep and placed close to the cheek-teeth, 

 is now shifted well forward and is transformed into a shallow groove, which in 



'- In more matured individual!?, the fissure in the alveohir border which lodged the canine cannot be 

 regarded as a true alveolus. 



'' Not only Diceraiherium but the Rhinocerata in general (such forms as the Amynodonts excepted) 

 undoubtedly developed the cutting incisors along the same general line. 



The result of the present study is contrary to the statement by Professor Marsh (Amer. Jour. Sci., 

 Vol. XIV, 1877, p. 251). It may be said here that the presence of the canines in the Amynodonts does 

 not prove "that the large lower teeth, usually regarded as incisors in Acerathcrium and many other 

 members of the Rhinoceros family, are really canines." 



