III. Limbs, skull, arches and dorsal vertebræ unknown, caudal vertebræ with fan-shaped neural and hæmal spines which may indicate relationship with _Ptyonius_ and _Œstocephalus_, but in those genera the ventral armature is weakly developed; ventral scutellæ curved rod-like plates arranged _en chevron_ with anterior angle, marked in abdominal region by distinct rounded pits _Ctenerpeton_ IV. Known only from fragments of the skull, teeth large and fluted; association in family provisional _Leptophractus_ V. Known only from a single interclavicle of peculiar form which resembles that of _Saurerpeton_; association in the family provisional _Eurythorax_ The members of this family are confined to the deposits of the Coal Measures at Linton, Ohio. _Ctenerpeton_, and possibly _Sauropleura_, were highly developed swimmers, but the strength of the limbs as exhibited, especially by Sauropleura and Saurerpeton, indicates that they had not entirely forsaken the land. =Genus SAUROPLEURA Cope, 1868.= Newberry, Proc. Phila. Acad. Nat. Sci., VIII, p. 98, 1856 (Pygopterus scutellatus). Cope, Trans. Am. Phil. Soc., p. 22, 1869. Cope, Geol. Surv. Ohio, II, pt. II, p. 402, 1875. Hay, Bull. U. S. Geol. Surv., No. 179, p. 419, 1902. Type: _Sauropleura scutellata_ Newberry. Vertebra and ribs well developed; limbs 4, rather large; 5 digits in the forefoot; carpus cartilaginous. Ventral armature of closely arranged rhomboidal scuta, arranged in lines, which are closely placed in chevrons, with the angle anterior. In none of the species of this genus have the usual 3 thoracic shields (clavicles and interclavicles) been observed. The abdominal scutæ, on the other hand, are much like those of _Saurerpeton_, being, however, smaller. The species formerly described by Cope in the genus _Colosteus_ are included in _Sauropleura_, where they find their closest allies. There are 7 species belonging in this genus: _Sauropleura digitata_ Cope, _S. newberryi_ Cope, _S. foveata_ Cope (_Colosteus_), _S. scutellata_ Newberry (_Colosteus_) (type of genus), _S. pauciradiata_ Cope (_Colosteus_), _S. longidentata_ Moodie, _S. enchodus_ Cope (_Anisodexis_). The species described by Cope as _Sauropleura latithorax_ is regarded as belonging to a distinct genus, _Saurerpeton_. =Sauropleura scutellata Newberry.= Newberry, Proc. Phila. Acad. Nat. Sci., p. 98, 1856 (_Pygopterus scutellatus_). Cope, Proc. Phila. Acad. Nat. Sci., p. 215, 1868. Cope, Trans. Am. Phil. Soc., p. 22, 1869. Cope, Geol. Surv. Ohio, II, pt. II, p. 402, 1875. Type: Specimen No. 8669 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. (Plate 14, fig. 3.)