A R C II /E T H E II I U M OF NEBRASKA. 659 



The crowns of the first and second true molars (Tab. xi., iig. 1) differ most 

 strikingly from those of the corresponding teeth of Ilyracotherium, in not possessing 

 a continuous ridge around iho base, and from those of Chwropotamus in the total 

 absence of a basal ridge on the inner side. 



The crowns of the teeth just alluded to of Archasotherium, are quadrilateral, 

 nearly cuboidal. The triturating surfaces anteriorly project into a remarkably pro- 

 minent basal ridge or platform one line thick, and three lines deep below the com- 

 mencement of the enamel. Posterior to this ridge the crown is elevated into six 

 raammillary eminences placed in two transverse rows a little convex forwards from 

 the middle, and smaller ones being placed in a trifling degree in advance of the others. 

 The postero-internal cone is continuous from its apex with an oblique prominent ridge 

 descending to the base of the postero-external cone posteriori}- . On the outside of 

 the crown is a slight oblique ridge connecting the bases of the external cones. The 

 apices of all the latter, which are unworn, exhibit an excavation relatively slight 

 to that of Ilyracotherium. 



The enamel where the original surface is preserved is thick and rugose. Upon 

 the second molar it is slightly worn, but upon the triturating surface of the iirst 

 has in greater part become very thin, and at the summits of the cones, excepting 

 ■the postero-external and internal, is completely removed, so as to present lenti- 

 cular-shaped surfaces of dentine. 



The crown of the posterior molar, judging from the base of connexion with the 

 fangs still remaining, Avas also quadrihateral, but the outside inclined backward and 

 inward as in Rhinoceros, and the posterior side was relativel}' small. 



The fragment alluded to containing two premolars, belonged to an older indi- 

 vidual than the preceding, and consequently the teeth are more Avorn. 



The last premolar has a quadrilateral crown, the inner side being the shortest, 

 and is convex. Posteriorly it possesses a prominent basal platform continuous 

 with a very slight ridge postero-externally. Antero-externally there is also a slight 

 ridge, which was probably continued anteriorly, but is worn away. The triturating 

 surface is worn off to a transversely ellipsoidal disk of dentine, margined by 

 enamel, and continuous, by a short isthmus on the anterior side of the tooth, with a 

 second and smaller transversely ellipsoidal surface just over the antero-external 

 fang. 



The premolar in advance is compressed, conoidal, convex externally and inter- 

 nally, and presents a slight basal ridge antero-internally and postero-externally. 

 The apex of the crown is worn off, leaving a subcircular dentinal surface, con- 

 tinuous Avith an exposed tract the Avhole length of the posterior side. 



The true molars and the contiguous premolars are implanted by three fangs, tAVO 

 external and nearly vertical, and a third internal, broad, and apparently composed 

 of tAVO Avhich are confluent. 



The penultimate premolar is implanted by tAVO fangs, also nearly vertical. 



The face in the specimen containing the true molars is very much fractured, but 

 in conjunction Avith the second fragment, a fcAV characters may be obtained Avhich 

 are important. 



