230 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.Y. 



Cgelodonta Bronn. 



G. etrusGus Falconer. Pliocene. Enrope. 



C. hemitceclms Falc. Postpliocene. Enroj)e. 



G. antiqiiitatis Blum. Postpliocene. Siberia ; Enrope. 



The above list embraces twenty-seven species, of wMch six are living. 

 There are probably several other distinct fossil species; but their char- 

 acters have not yet been sufficiently made known to enable me to refer, 

 them to their proper places. It will be observed, from the preceding 

 catalogue, that eight species have been found in IS^orth American for- 

 mations, ten in European, and three in those of Hindostan. It ap- 

 pears also that no extinct species of the true genus of Bhinocerus has yet 

 been found in Korth America or Europe, and that no extinct rhinoceros 

 of Korth America which is known, jiossessed a median dermal horn. 



It can readily be seen that the genera above defined form a graduated 

 series, the steps of which are measured principally by successive modi- 

 fications of four different parts of the skeleton. These are, first, the 

 reduction of the number of the toes of the anterior foot ; second, the re- 

 duction in the number and development of the canine and incisor teeth ; 

 third, the degree of closure of the meatus auditorius externus below; 

 and, fourth, in the development of the dermal horns of the nose and its 

 supports. While these characters have that tangible and measurable 

 quantity which renders them available for generic diagnosis, there are 

 others which possess a similar significance, and which I now notice, so 

 far as they are observable in the extinct species of Korth America. 



I premise by observing that the Aceratheria of this continent have only 

 been found in the eastern and western divisions of the White Eiver 

 formation, while the species of ApJielops are confined, so far as is known, 

 to the Upper Miocene or Loup Eiver formation. 



The posttympanic process is, it is well known, well separated from 

 the postglenoid process in the tapir, so as to leave the auditory meatus 

 widely open below. The arrangement is similar in Hyracodon. In 

 EMnocerus, as shown by Elower, the meatus is closed below by the 

 coosification of the two processes. In the oldest genus of the family 

 Aceratherium, the relations of the parts are as in Hyracodon. In Aphe- 

 lops the two processes approach each other, but do not come in close 

 contact as in the genus GeratorJiinus. 



The postglenoid process is low and transverse in the tapirs ; in Bhi- 

 nocerus it is long and has a triangular section. In some species of Amer- 

 ican Aceratheria its form is much like that of the tapirs {A. mite, A. occi- 

 dentale) ; while in others {A. pacificum) and in the species of Aphelops the 

 form of this process is as in Bhinocerus. 



In the tapirs, the foramina sphenoorbitale and rotundum are distinct. 

 They are also distinct in Aceratherium Quite. In A. pacificum they are 

 confluent, but the walls of their orifice present two opposite projections, 

 which are the rudiments of a dividing septum. In Aphelops these 



