DAKOTA AND NEBRASKA. 205 



witbin the rounded angle postero-internallj' a moderately large tubercle. In front of 

 the latter the basal ridge is more or less tuberculate; behind, it is more even and con- 

 tinuous, and has an acute edge. The inner slope of the crown is most prominent 

 just in advance of the middle, and in front is somewhat depressed. The summit and 

 posterior border of the crown are slightly worn. 



The lower jaw of Hijopotamus hovlnus possesses a continuous row of six molars, and 

 has the first premolar removed from the others by a wide interval. The specimens 

 above described would appear to indicate that the first premolar was not separated 

 from the others in the animal to which they belonged. This fact, coupled with the 

 one that the only specimen of an inferior molar we have the opportunity of examin- 

 ing presents slight difierence from the corresponding tooth of H. bovinics, may indicate 

 the American animal as pertaining to a genus different from either Hyopotaimis or 

 Anthracotherium. 



The specimen of an inferior molar, figures 5, 6, consists of the crown of the last of 

 the series, without its hinder or fifth lobe. It nearly resembles the corresponding 

 portion of the same tooth in Hyopotamus hovinus. It is composed of two transverse 

 pairs of pyramidal lobes, of which those external are considerably the larger, and 

 they embrace the internal ones. The latter are comparatively narrow, convex exter- 

 nally, and devoid of all trace of a basal ridge. They present an antero-external sub- 

 acute ridge descending to the interspace separating them from the external lobes. 

 The ridge of the postero-internal lobe is the more prominent, and is more external. 

 The antero-internal lobe presents another sub-acute ridge posteriorly, the correspon- 

 dent of which is undeveloped in the lobe behind. The external lobes of the crown 

 are angularly convex outwardly, slightly depressed in front and behind, and deeply 

 separated by the intervening valley. From their summit acute ridges diverge out- 

 wardly, and embrace the bases of the internal lobes. The anterior ridge of the 

 antero-external lobe descends in front of the base of the contiguous internal lobe, 

 and there appears to have ended in a prominent tubercle, which is broken off in the 

 specimen. The posterior ridge of the former lobe produces a transverse festoon be- 

 tween its summit and that of the latter one. The anterior ridge of the postero- 

 external lobe descends to the interval of the internal lobes, and then bends forward, 

 outward and upward to the festooned ridge between the anterior pair of lobes. The 

 posterior ridge of the same lobe descends behind the base of the postero-internal lobe, 

 and there appears to have become continuous with the ridge of the posterior lobe, 

 which is lost in the specimen. A thick indented basal ridge occupies the fore part of 

 the crown, but not elsewhere, except as a tubercle at the exit of the transverse 

 valley. 



The measurements of the specimens referred to Hyopotamus americanus are as 

 follow : 



