The present species adds another form to the diversity of structure presented by the Carboniferous Microsauria. It is closely allied to _Erpetosaurus_ (_Tuditanus_) _obtusus_ Cope, from the same beds (Linton, Ohio), but differs from it especially in the position and shape of the orbits and the acute form of the skull. Other characters which amount almost to generic significance are found in the posterior prolongation of the frontal and in the triangular form of the skull. Only the skull of the animal is preserved. The character which is common to all members of the genus _Erpetosaurus_, the cranial rugosity, is present in this species on the squamosal and supratemporal. This character alone would not, however, suffice to separate the form generically, but the general morphology and arrangement of the cranial elements is such that reference to any other genus save Erpetosaurus would not be possible. The skull of _E. acutirostris_ takes the form of a rounded triangle. Its base is some 50 mm. in extent, and this width gradually narrows to 31 mm. across the orbits and still more towards the snout. The form of the skull is not widely different from that of the type species, _E. radiatus_ Cope, but the differences are sufficiently apparent. Nearly all the elements of the cranium can be detected (fig. 22, D). The bony portion of the cranium has nearly all been lost, leaving only the impression; and the matrix in which the skull was embedded has been forced up into the sutures between the cranial elements, thus forming ragged ridges where the bones of the skull joined.