E. L. Troxell — Homed Eocene Ungulates. 35 



of the horn-supports seen in the modern rhinoceroses, 

 bnt here the horns, when there are two of them, are placed 

 one in front of the other. 



It is well known that Colonoceras agrestis had similar 

 thickenings of the nasal bones, even more rugose ; they 

 probably had a common ancestry, but that species is far 

 separated from the present one in other respects : C. 

 agrestis is about two thirds the size; the premolars have 

 the simple internal cone ; the strong transverse ridge on 

 F 3 is anterior and not medial in position ; and finally the 

 rugose areas are midway on the nasals and not posterior 

 as in the new species. 



Detailed Morphology of M. bicomutus. — The upper 

 incisors increase in size from' front to rear, and there is 

 a progressively larger space between each and between 

 the last incisor and the canine. The third incisor is 

 subcaniniform, being rather long and slightly recurved, 

 but it is narrow transversely. The canine is only 

 moderately long; it is compressed and recurved and 

 therefore bears no resemblance to those of the later 

 rhinoceroses except Hyracodon Leidy. 



The first premolar, although elongated fore and aft, 

 is much broader than that in Hyrachyus. This tooth 

 has the one main protocone and on the inner side an inci- 

 pient deuterocone with minor ridges. The second pre- 

 molar is nearly circular in form, consisting mostly of 

 the large strong outer cone (protocone), with a small 

 tritocone behind it, and of one prominent inner cone, the 

 deuterocone. A thin ridge runs from the latter to the 

 middle of the ectoloph and divides the tooth equally; 

 this is a distinctive feature of the genus. 



Undoubtedly the most important feature of the skull 

 is the double inner cone on each of the larger premolars. 

 It shows the beginning of the separation of the tetarto- 

 cone from the deuterocone, and a first step toward the 

 assumption of the molariform condition which was 

 actually realized both in Flyracodon cf. H. leidyanus, 

 and in Ccenopus cf. G. tridactylus metalophus. This 

 partial separation is brought about by a vertical groove 

 on the inner side of each tooth which, theoretically, is the 

 predecessor of the normal, transverse, median valley. 

 There is a small internal basal cingulum cutting across 

 the base of this groove on P 4 . 



