This species exhibits considerable variation, both in size and proportion of the teeth. We have two specimens of it, one of which agrees exactly with the measurements given by Professor Cope, while the other is stouter, and probably belonged to an old male. The incisors are too much broken for description. The canine has a long, stout fang, which is subcircular in section; the crown is compressed, and shows a distinct cutting edge posteriorly. It is short, and tapers rapidly; in this respect differing from _Notharctus_, Leidy, which has a long recurved canine. The first and second premolars are inserted each by a single fang; the third and fourth by two. The third premolar consists of a simple conical crown with a small posterior heel; and the fourth has this heel enlarged, with a small tubercle developed inside and slightly behind the principal lobe. All the premolars have striated enamel, and very feebly marked basal ridges. The true molars are considerably larger than the premolars; the third is the longest of the series, and the second is the widest. They all seem to be inserted by two fangs. The posterior fang of the last molar is a flat quadrate, of the same size throughout, having a great fore-and-aft diameter, and apparently no nerve cavity. The mandible is strong but shallow, has a curved alveolus and lower margin, and the teeth form a curve with convexity outwards. The symphysis is short and oblique. This genus presents a close resemblance to the modern _Lemur_, but at the same time shows several differences. We may give these differences in systematic order: (1) Greater number of premolars, in _Tomitherium_ = 4, in _Lemur_ = 2. It will be observed, however, that the first and second premolars of _Tomitherium_ are very small and single-rooted, and that their disappearance is a comparatively slight change. (2) The canines are subcircular in section, not nearly so much compressed. (3) Greater breadth of the molars in proportion to their length. (4) Two internal cusps on the molar. (5) Much greater size of the last molar. (6) Greater depth and thickness of the jaw. (7) Greater curvature of alveolus and lower margin of ramus. (8) Longer and more oblique symphysis. The third and fourth premolars of _Tomitherium_ correspond almost exactly to the first and second of _Lemur_, but they are not quite so high and sharp. The interior tubercle of the second premolar is not so distinct in the latter genus. In the second specimen the cusps are all low, and the crests prominent, giving the molars something of the appearance of _Opisthotomus_. This difference is probably sexual. _Measurements._