E. L. Troxell — Ccenopus, the Ancestral Rhinoceros. 49 

 Ccenopus tridactylus avus, subsp. nov. 



(Fig. 5.) 



Holotype, Cat. No. 10251, Y. P. M. Upper Oligoeene (Protoceras beds), 

 South Dakota. 



Ccenopus tridactylus avus, subsp. nov., is very probably 

 a true form of this species, but as a subspecies shows a 

 decided trend toward Dicer atherium armatum (see fig. 6.) 

 The type material consists of the skull of a young 

 animal still retaining the last milk tooth, Dp^, and having 

 the last molar uncut. The first three premolars (P^ is 

 also uncut) are notable for the parallel, separate lophs, 

 uniting only with wear. The metaloph is at first longer, 

 but because of the extended base of the deuterocone, the 

 protoloph soon surpasses it in length. P^ has two paral- 

 lel, backward curving crests, but the metaloph does not 

 encircle a lake (postfossette) as does Osborn's paratype 

 (1898, pi. 13). On the larger premolars, small sharp 



Fig. 5. — Ccenopus tridactylus avus, subsp. nov. Holotype. Premolars 

 and first and second molars. X 1/3- 



folds appear on the ectoloph, and on P^ (left) one has an 

 unusual position behind the metaloph. On P^ and P^ 

 (right), there is a fold in the anterior angle between the 

 two lophs. 



Cristae and crochets may be seen on the molars, and the 

 latter, especially on M^, have encroached so far on the 

 median valley as to isolate a small lake (medifossette). 

 On the protoloph of M^, two vertical valleys, anterior and 

 posterior, partly separate the protocone from its proto- 

 conule ; the posterior valley emphasizes and sets off the 

 moderate antecrochet. The inner cingula are broken 

 across the bases of the protocone and hypocone, and are 

 peculiarly offset where the two ends join in the median 

 groove. 



Am. Jour. Sci., Fifth Series, Vol. II, No. 7 —July, 1921. 

 4 



