Peterson: Skeleton ok DicEUATHKRir.M Cooki Peterson. 27ri 



cned cctolopli, wliilc t)ii M '' both crotchet and crista are unite plain. 

 The diameters of the teeth of the jiresent slcull are only very slightly 

 greater than those in the type, in fact there is practically no difference 

 in all the nuMsurenirnts of the two skulls here coniiiarcd when an al- 

 lowance is gi\en for the slightly larger size and crushing of No. 2499. 

 A curious feature of some individuals is seen in the presence of the 

 upper canine. This feature is, however, not to be regarded as of any 

 great morphological importance and may be regarded as a case of 

 atavism, pointing back to the Oligocene forms {Leptaceratheriiim and 

 Accrathcriiim^) which ha\e the superior canines much more strongly 

 developed. The premaxillaries are restored from other individuals. 



The lower jaws are quite heavy, the transverse diameter of the 

 horizontal ramus being thick and the diastema between the large 

 incisor and P^ rather short. The inferior border of the ramus ter- 

 minates posteriorly in a heavy and excessively everted process, which 

 is a characteristic feature of the angle of the lower jaw of old indi- 

 viduals (especially males) of this species. The glenoid condyle is very 

 broad transversely and the coronoid process has rather an excessive 

 forward direction. The median incisors are not present, but are 

 plainly indicated by alveoli. The lateral, or cutting, incisors are much 

 worn, as is also the case with the molar pre-molar series. 



The deciduous dentition in all the different stages is well repre- 

 sented in the material of Dicerathenum from this fossil quarry. In 

 this connection it may be interesting to say a few^ words in regard to the 

 deciduous low-er incisors and canines. From the material in the 

 Carnegie Museum there have been selected three individuals of a 

 slight difference in ages in order to illustrate my point. In each case 

 the lower canines or their alveoli are present and in two of the three 

 pairs the full series of incisors are represented; the third pair being 

 farther advanced in maturity. 



In the valuable monograph on the "Extinct Rhinoceroses" by Pro- 

 fessor Henry F. Osborn he has distinguished the Rhinocerotidai as 

 follows: "Manus functionally tridactyl. Upper canines atrophied. 

 Median upper incisors and lower canines opposed and irregularly 

 de\eloped" (/. c, p. 80). In lilter publications Lucas and Hatcher^ 



^Osborn, H. F., "The Extinct Rhinoceroses," Mem. Amer. Miis. Nat. Hist. 

 Vol. I, pp. 132, 146, 1898. 



*Proc. National Museum, Vol. XXIII, No. 1207, pp. 221-223; Ann. Carn 

 Mus., Vol. I, 1901, pp. 135-144. 



