^0.2.] COPE OX EXTINCT RHIXOCERID^. 231 



foramina are one, as in Eliinocerus. At the same time, the external 

 wall of the alisphenoid canal is shorter and thinner in the Aceratheria 

 than in the Aplielopes. 



In the older tj-pes of Perissodactyla, e. g. Symhorodon, the foramen ovale 

 is situated well in advance of the foramen lacernm medius, and is sepa- 

 rated from it by a considerable space of the sphenoid bone. The same 

 structure is seen in Hyracodon and in Aceratherium. In Ai)1ielops, the 

 foramen ovale approaches near to the f lacernm, so as to be separated 

 by a narrow bridge only in A. megalodiis, which is wanting on one side 

 in a specimen of A. malacorkimis. In the genus Ehinocenis, these fora- 

 mina are not di\ided. 



In the structure of the teeth, the same serial order is to be observed. 

 Commencing with the incisors f in the tapiroid tj^pes and Hyracodon, 

 and canine -I", we findf fin Zalahis; \^mAceratlierium;~^ ^^mAphelops; 

 I f in CeratorMniis and EMnocerus, to ^^ ^ in Atelodus and Ccelodonta. 

 As to the molars, in those of the ux)per jaw the series of modifications 

 consists of successive complication of the transverse crests. In Hyraco- 

 don, as in the tapiroid genera, the external wall of the posterior molar is 

 continued beyond the posterior cross-crest ; in the Ehinocerida^, gener- 

 ally the external wall is not continued beyond this crest, but is in line 

 with the posterior cross-crest. In a specimen of Aceratherium o'ceidentaJe, 

 the posterior superior molar of one side is like that of Hyracodon, while 

 that of the other side is like that of EMnocerus. The cross-crests in 

 Aceratherium are quite simple, having slight bulges into the median 

 valley. In the species of Aphelops these bulges are more prominent, es- 

 pecially that of the posterior crest, which is more externally s; tuated 

 than that of the anterior cross-crest, so that the fundus of the valley is 

 turned abruptly backwards. In several of the existing species, this 

 bulge becomes an antero-posterior crest, and the fundus is further di- 

 vided by other crests from the outer wall and elsewhere. The cingula 

 become so elevated as to cause an isolation of the valleys as fossae at a 

 comparatively early stage of wear. This state of things commences in 

 the extinct species of Kansas, the Aphelops fossiger. 



In the bones of the skeleton, modifications accompanying those of the 

 cranium and dentition may be observed. The femur of the species of 

 the earlier formations may be readily distinguished from that of those 

 of the later Tertiaries by the forms of both the extremities. In the Ace- 

 ratheria this bone resembles that of the tapirs in the form of the great 

 trochanter. This process is produced at its external border, has a 

 recurved apex, and encloses a deep trochanteric fossa. In Aphelops it is 

 precisely as in EMnocerus, obliquely truncate externally, without promi- 

 nent apex or well marked fossa. In the Aceratheria the inner crest of 

 the rotular groove is but moderately prominent; in Aphelops and Ehi- 

 nocerus it is greatly developed. 



The succession of development of the line of the Ehinocerida> is now 



