Type: Specimen No. 10850, American Museum of Natural History. Collected by Dr. J. S. Newberry. Horizon and locality: Linton, Ohio, Coal Measures. [Illustration: Fig. 36. Mandible of _Leptophractus dentatus_ new species, from the Linton, Ohio, Coal Measures. × 1. Original in American Museum of Natural History.] The type is a single right mandible, nearly entire, of a rather large animal. The specific characters for the separation of the new form from the previously described _L. obsoletus_ and _L. lineolatus_ are the smaller size and slenderness of the mandible, associated with uniform teeth, which are slender and delicately fluted. There are 17 teeth preserved, the largest of which is 8 mm. in length. From the posterior tooth the series gradually descends to half this length 0.5 inch from the anterior end of the mandible. The exact form of the mandible can not be determined, but so far as can be seen it is very slender, coming almost to a point at the anterior end. The posterior portion is wide, but apparently not very heavy. There is a fragment associated with the specimen which discloses a few teeth, but its position in the cranium can not be determined. Measurements of the Type of Leptophractus dentatus Moodie. mm. Length of mandible, as preserved 80 Anterior width of mandible 3 Posterior width 28 Length of most anterior tooth 4 Length of tenth posterior tooth 7 Width of this tooth at base 3 =Leptophractus lineolatus Cope.= Cope, Proc. Am. Phil. Soc., XVI, p. 576, 1877. Type: Specimen No. 1088 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. (Plate 22, fig. 1.) Cope's description of this species, to which I have nothing to add, is as follows: "This species is based on portions of the skull of two individuals of large size. Both upper and lower jaw with teeth are represented and the teeth are of very large size. The deepest element preserved has been provisionally referred to the mandible and will be so described. This element bears two types of teeth in very heterodont fashion. The teeth in the back portion of the jaw are rather short and slender. The teeth more anteriorly are, some of them, very long, rather stout, with their bases longitudinally fluted, as are they all v The longest tooth in the jaw measures slightly less than an inch. The bone is not well preserved, but seems to have been ornamented with grooves of no great depth. The most anterior teeth of the jaw are smaller than the posterior ones.