The only known specimen of this anomalous amphibian is incomplete, representing the posterior half of the skeleton, and an abundance of ventral scutellæ or calcified myocommata. The block of coal containing these interesting remains is from Linton, Ohio, and is preserved in the geological collections of Columbia University, from which institution Professor Grabau very courteously forwarded it for study. _Ichthycanthus platypus_ was described by Cope from the Linton, Ohio, Coal Measures, locating it doubtfully in the Permian genus _Eryops_ on account of the unusual condition of the tarsus and reconsidering a former decision in favor of a Coal Measures genus _Ichthycanthus_. In this disposition of the species into the Permian genus he is followed by Hay (317); but Baur (28) regarded the form as a member of the Coal Measures genus _Ichthycanthus_, after commenting on the later definition by Cope. The type of the genus, _Ichthycanthus_, to which Cope first allied the species under consideration, is _I. ohiensis_, a supposed amphibian from the Coal Measures of Linton, Ohio, founded on incomplete material. The form combines in an unusual and remarkable degree reptilian and amphibian characteristics. The leg bones, pelvis, and tarsus are all strikingly reptilian, but the phalanges in the arrangement of elements are so typically amphibian that if we had no other means of diagnosis we would incline to locate this Coal Measures species among the Amphibia. The leg (plate 23, fig. 1) recalls in its structure that of another Coal Measures species, _Eosauravus copei_ Williston, which is, however, clearly a reptile. While there is a general degree of similarity between the foot structure of _Eosauravus copei_ and _Ichthycanthus platypus_, yet there are very great differences in the phalangeal formula and the arrangement of the tarsal elements. These differences are clear and indicate a separation of the two species into distinct classes. The phalangeal formula in the _Eosauravus_, 2-3-4-5-4, is typically reptilian; while in the _Ichthycanthus_, 2-2-3-3-3, it is amphibian. The tarsus of the _Ichthycanthus_ is amphibian in the presence of an intermedium, but this is very small and the remaining tarsal structures have nothing which might not be found in an early reptile. There may be a single or even two centralia in the reptilian tarsus among the early forms.