"The inferior lower molars do not display a bifid or double anterior cusp; and the crowns exhibit two anterior cones, and an inner cone and outer crescent posteriorly. The posterior crescent is well defined, and is continued on a narrow crest to the anterior inner tubercle. The posterior molar presents the peculiarity of a series of three cusps in one line, the median having another or lateral cusp near it." This genus has hitherto been found only in the Wahsatch formation; but we have discovered it to be represented in the Bridger series by the species _O. astutus_, Cope. Our specimen consists of a part of the ramus mandibuli containing a single molar tooth. CARNIVORA SINOPA, Leidy. A genus of small carnivorous animals, which Dr. Leidy regards as intermediate between the recent _Canis_ and the extinct _Hyænodon_. Owing to the fragmentary condition of the remains found, no satisfactory generic definition has been given. From the portion in our collection, we are able to throw some further light upon the genus, summing up the generic characteristics thus: Small carnivores, which have the last upper premolar as sectorial (thus differing from _Hyænodon_), the other premolars simple and conical. The sectorial is shorter, antero-posteriorly, than the preceding tooth; has a short blade of a single lobe, and a large cusp developed from the posterior part; a cingulum surrounds the entire crown. The _lower_ sectorial has the blade of a single lobe, and with a short heel. Sinopa rapax, Leidy. Proceedings of Ac. Nat. Sc., 1871, p. 115. In addition to the molars of the lower jaw, described by Dr. Leidy we have what corresponds to the third and fourth premolars of the fox, their dental formulas being probably the same. The third premolar is small and pointed; differing from the corresponding tooth in the fox, (1) in its being less compressed, (2) in its shorter antero-posterior diameter, (3) in the straighter and more nearly equal margins, and in (4) the absence of a posterior heel. The tooth is inserted by two fangs, as in _Canis_ and _Hyænodon_. The posterior shows a rudiment of a third, which is connate with its entire length above the alveolus. There is an indistinct cingulum around the entire crown. The fourth premolar has a very curious shape. The blade of this tooth resembles the crown of the third, but is smaller. It is inserted by three fangs, the disposition of which is opposite to that in _Canis_, the internal, being on the same transverse line as the posterior external, instead of the anterior, as in _Canis_. From the internal fang arises a sharp cusp, which is nearly as large as the blade of the tooth, the two are connate at base. The anterior face of the crown is much worn, and there is a small anterior heel formed by the basal ridge. The cingulum is complete all around.