It has seemed necessary to propose a new family for the reception of the single species _Macrerpeton_ (_Tuditanus_) _huxleyi_ Cope. The characters exhibited by this species are so different from those offered by other members of the Carboniferous Microsauria that it is clearly distinct. In its cranial characters and the position of the orbits it approaches most nearly to _Eryops megacephalus_ Cope from the Permian of Texas. In some of its characters the present form shows a similarity to _Dasyceps bucklandi_ Lloyd (324), from the Permian of Kenilworth, England; more especially is this similarity found in the form of the skull, the size and shape of the teeth, and the posterior position of the orbits, and their wide removal from the border of the skull. Only a fragment of the skull has hitherto been known, but repeated study of this fragment (123) has disclosed the wide diversity (462) of its characters. An almost complete skull, described below, substantiates the characters based on the fragment. Another species is here added to the genus, based on a portion of a mandible and a portion of the skull. The family, Macrerpetidæ, may be defined (465) as follows: Skull larger than in any other known microsaurian, unless Baphetes proves to be microsaurian; cranial elements sculptured with pits and coarse grooves; lacrimal element present, teeth large, curved inwards and fluted; mandible heavy; orbits located far back on the skull and near the median line so that the interorbital space is about half the space from the outer edge of the orbit to the border of the skull, thus approaching the condition known in Eryops; the ribs (?) are strong, heavy, and curved, with an incipient tubercle. =Genus MACRERPETON Moodie, 1909.= Moodie, Jour. Geol., XVII, No. 1, pp. 72-74, fig. 17, 1909. Type: _Macrerpeton huxleyi_ Cope. The genus _Macrerpeton_ was proposed for the reception of the amphibian species described by Cope as _Tuditanus huxleyi_ (123). This form he placed provisionally in genus _Tuditanus_, since it seemed to present the same type of sculpturing of the cranial elements similar to that found in _T. radiatus_ Cope. But this species has been removed from Tuditanus and placed in a new genus, _Erpetosaurus_ (462). A close study of the type specimen of _Tuditanus huxleyi_ Cope shows (465) great variation and distinction from any of the species described from Linton, Ohio, or indeed from any Carboniferous form thus far known.