Elongate, slender, weak-limbed, aquatic microsaurians. Neural and hæmal spines of vertebræ elongated, expanded and sculptured. Ventral armature weakly developed or absent. Skull lanceolate, with long, slender teeth. Three genera are assigned to this family: _Ptyonius_, _Œstocephalus_, and _Thyrsidium_. The forms are very closely related, and when additional material is secured the three genera may be found to be identical. The species included in this family are: _Ptyonius pectinatus_ Cope, _P. vinchellianus_ Cope, _P. marshii_ Cope, _P. nummifer_ Cope, _P. serrula_ Cope, _Œstocephalus remex_ Cope, _O. rectidens_ Cope, _Thyrsidium fasciculare_ Cope. The species are all exclusively from the Linton, Ohio, Coal Measures, and most of them are known from abundant material. =Genus PTYONIUS Cope, 1875.= Cope, Geol. Surv. Ohio, II, pt. II, p. 373, 1875. Cope designated no species as the type, but we may regard _Ptyonius pectinatus_ as typical. Form elongate, with long tail and lanceolate cranium. Limbs weak, a posterior pair only discovered. Three clavicular elements; abdomen protected by packed osseous rods, which are arranged _en chevron_, the angle directed forward. Neural and hæmal spines of caudal vertebræ expanded and fan-like. Ribs well developed. The various species vary in length from 3 to 10 inches. They are the most abundant amphibian in the Linton beds. The present genus resembles _Lepterpeton_ Huxley (334), of the Kilkenny, Ireland, Coal Measures. But that genus possesses divided abdominal rods, or "oat-shaped scales," and the form of the cranium and proportions of the body are different. The genus is closely related to, possibly identical with, _Œstocephalus_, but additional material will be required to settle this point. Cope (123) gives the following key for the separation of the 5 species: x. Abdominal rods coarser, not more than 10 in 5 mm. Median pectoral shield discoid, radiate-ridged; muzzle short _P. nummifer_ Median pectoral shield oval, pitted and ridged _P. marshii_ xx. Abdominal rods hair-like, 15 or more in 5 mm. Median pectoral shield with radii from the center, the principal forming a cross; form wider _P. vinchellianus_ Middle pectoral with pits at the center and few or no radii; form narrow _P. pectinatus_ Middle pectoral shield narrow, closely reticulate medially, and radiate towards the circumference; size half that of last _P. serrula_ =Ptyonius pectinatus Cope.= Cope, Proc. Acad. Nat. Sci., 1868, p. 216. Cope, Trans. Am. Phil. Soc., XIV, p. 20, 1869. Cope, Trans. Am. Phil. Soc., XV, p. 266, 1874. Cope, Geol. Surv. Ohio, II, pt. II, p. 377, pl. xxvii, fig. 7; xxviii, figs. 2, 3, 6; pl. xxix, fig. 2; pl. xxx, fig. 2; pl. xxxv, figs. 1-3; pl. xli, fig. 1, 1875. Moodie, Proc. U. S. Nat. Mus., 37, p. 24, pl. 8, fig. 3, 1909.