No.2.\ COPE OX EXTINCT RHINOCEKID^. 237 



In its form it must have been mucli like a Hipiiopotanms. Its limbs, 

 and especially the feet, were very robust. 



The five species may be distinguished by the characters of their supe- 

 rior molar teeth as follows : 



I. Fundus of the transverse valley divided by processes. 



Posterior marginal fossa open ; transverse crests, the posterior with a long 

 anterior process, the anterior with a long posterior process A, cmssus. 



II. Fundus of transverse valley without jtrocesses. 



a. Posterior marginal fossa oj>en, excejit in advanced wear. 



Posterior cross-crest with a strong anterior process ; ];)remolars with a cingu- 



lum extending round the j)osterior cross-crest A. malacorhlnus. 



Posterior cross -crest with strong anterior process ; no cingulum on posterior 

 cross-crest of premolars; internal cingulum of true molars; smaller. 



A. meridiami.s. 



Posterior cross-crest. with little or no anterior process; cingulum wanting 



from posterior cross-crest of premolars and from inner side of true molars ; 



larger A. megalodus. 



aa. Posterior marginal fossa enclosed by a high posterior cingulum. 



Strong anterior jirocess of j)osterior cross-crest and posterior process of an- 

 terior cross-crest ; no cingulum on true molars ; large A. fossigeK 



Position. — The longest known species, the A. crassus, was found by 

 l)r. Hayden on the Mobrara Eiver, Nebraska. Teeth presenting the 

 same characters have been found in IS'orthern Kansas and Eastern Colo- 

 rado. The other species are more restricted geographically. A consid- 

 erable exiDloration in the Loup Fork beds of Northeastern Colorado, 

 conducted by myself in 1873, yielded four individuals of A. megalodus^ 

 but no fragments referable to the other species. Explorations in Xorth- 

 ern Kansas produced three individuals of A. fossiger and four of A. 

 malacorhinus, but not a fragment of A. megalodus. 



History. — In my original definition of this genus, I relied on the num- 

 ber of premolars in distinguishing it from BMnocerus, as weU as on the 

 absence of the horn. These teeth are generally f in Aifhelops^ and are 

 said to be f ill BMnocerus, in most works on the subject. These num- 

 bers are not constant; on one side of both jaws in Ai)helops from Colo- 

 rado I have observed a first premolar, and on one side of the upper jaw 

 of A, malacorliinus there are four premolars ; the other side is injm^ed. 

 In several species of BMnocerus, three premolars only are usually found 

 in the mandible. I may add that Lesson and Peters* have described 

 a BMnocerus inermis Less., which is found living on some of the islands 

 at the mouth of the Ganges. The only known specimens are the skulls 

 with portions of the skin, of a female and young. These are hornless, 

 and in general structure allied to the B. sondaicus, yet presenting some 

 important differences.t Should the characters of this form prove to be 

 specific, and the male be found to lack the horn, it must be regarded as 

 a species of A^lielops. 



'^Monatsberichte Berlin. Akademie, 1877, p. loS, pi. 1-2. 



t Peters rei^resents the posttympanic as not coossified below the meatus as in B. son- 

 daicus. 



