PEEISSODACTYLA. 635 



no more perfect specimens having been received since that publication was 

 issued, I do not repeat, but simply refer to it. 



A lower jaw from the Big-Horn Basin may belong to this species, or 

 may possibly be a new sjDecies of PUolophus. The uncertainty is due to 

 the fact that the crowns of only two molar teeth remain, one or both of 

 which may be premolars. Supposing the anterior tooth to be the fourth 

 premolar, it is shorter than the corresponding tooth in H. vasacciense. 



The bases of the second and third j^remolars are of about equal length, 

 and each is considerabl}- shorter than that of the fourth premolai-. The 

 crown of the third premolar is remarkable for the size of its internal tubercle, 

 which is more robust at the apex than the external. Its position is very 

 little posterior to the latter. It is unlike the latter in ha^ang no ridge de- 

 scending to the anterior base. There is no distinct anterior basal tubercle. 

 The tubercle of the heel is well developed, and sends a crest forward to 

 the middle of joined anterior tubercles. On the internal side of the heel 

 there is a rudimental internal tubercle. No external cingulum. The fourth 

 premolar has a strong external and posterior cingulum, the latter culmina- 

 ting in a median posterior tubercle. 



Measurements. m. 



Length of Pm. iii and iv Oil 



Length of M. i 007 



Width of M. i posteriorly 0045 



Depth of ramus at M. i 018 



Besides the above a second specimen, a part of a lower jaw, was sent 

 from the Big Horn 



Hyracotherium venticolum Cope. 



Bulletin U. S. Geological Survey Terrs., VI, 1881, p. 198. Hyracotherium vasacciense Cope. American 

 Naturalist, 1880, p. 747; not of jirevious publications. 



Plates XLIXrt, XLIX 6, and XLIXc. 



Repi'esented by an entire skull, with many bones of the skeleton, of 

 one individual, and the mandible of a second. 



In general, this species is to be distinguished from its near ally, the 

 H. vasacciense, by the slender mandibular ramus. The depth of this bone 

 is about equal to that found in the larger varieties of the H. angustidens, but 

 the teeth are much larger, having the proportions of those of the H. vasac- 

 ciense. This remark applies especially to the last inferior molar. 



