Troxell — Diceratherium and the Diceratheres. 203 



The mure is entirely absent from P* of D. armatum and 

 is incipient on P^-^. It is a feature of varying develop- 

 ment in CcBfiopus and Metaccenopus of the Great Plains '^- 

 also but is not homologous with the encircling protoloph 

 of more primitive forms. 



Premolar teeth seem to reflect most quickly the evolu- 

 tionary changes of a race. Here are shown the crista 

 and crochet, supplemented by numerous small folds giv- 

 ing a distinctive air of advancement, features which were 

 not simply lost by wear in D. armatum but which never 

 existed. 



Both molars and premolars of D. armatum have deep 

 postfossettes. In the new species, these are shallow and 

 narrow and the metaloph rises from the posterior cingu- 

 lum direct. The teeth have the appearance of being 

 slightly longer-crowned than usual, and they have decid- 

 edly straighter outer surfaces. In D. armatum there is 

 a deep groove, especially on P^ just in front of the para- 

 cone, and a second distinct ridge in the middle of the 

 ectoloph. 



The development of the crochet and antecrochet in the 

 first molar results in a closing of the median valley and 

 the raising of its floor a full centimeter above the deepest 

 part. This is not homologous with the mure of the 

 premolars. In M^ of D. armatum there is scarcely a 

 change of level of the valley bottom. 



The cingula are almost obsolete on the outer side of 

 all teeth, but one is present about the protocone of each 

 molar. There is a coating of cement on the outer sides 

 of the cheek teeth. 



Whether or not this specimen had heavy nasals for the 

 support of horns we have no way now of determining, but 

 it is of minor importance since both types are well known 

 among the John Day diceratheres and it is considered a 

 sexual variation simply. 



A very large antorbital foramen was traversed by the 

 blood vessels and nerves to the lips and face; indicating 

 that there may have been a large facile lip like that of 

 the African black rhinoceros of to-day. 



Am. Jour. Sci. — Fifth Series, Vol. IT, No. 10.— October, 1921. 

 14 



