The species is represented by the opposite halves of a single specimen, which includes only the cranium and anterior half of the body. The fan-shaped neural spines commence but a short distance behind the line of the pectoral shields. They are low, with a few coarse ridges, the margin being entire. The abdominal rods are delicate and hair-like. The interclavicle is oval, with a few radiating crests, which originate at the center; in the areas behind there are a few scattered tubercles. The clavicles are ridged near the margin. The cranium is lanceolate in form, and the bones of the dorsum are marked with a few raised points and ridges. The species is about the size of _Ptyonius pectinatus_ Cope, and differs, apparently, from that species in the rather insignificant character of a narrower interclavicle and in the ornamentation of the same. Dedicated to Professor Alexander Winchell, of the University of Michigan. Measurements of Ptyonius vinchellianus Cope. mm. Length of cranium 20 Width of same 8 Length of interclavicle 4-2 =Ptyonius marshii Cope.= Cope, Trans. Amer. Phil. Soc., XIV, p. 24, 1869 (_Colosteus marshii_). Cope, Geol. Surv. Ohio, II, pt. II, p. 375, pl. xxvii, fig. 6; pl. xxviii, fig. 3, 1875. Cope, Proc. Am. Phil. Soc., XII, p. 177, 1871. Type: Specimen No. 1157 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. The head is elongate lanceolate. The upper surface of the frontal bones is punctate-rugose in relief, with short radii toward the margin. The distal two-thirds of the mandible is narrow wedge-shaped; the external surface is coarsely pitted. There are no teeth preserved. The pectoral elements are displaced, but the clavicles are subtriangular, and are strongly ridged toward the inner margin. The interclavicle is short spatulate, the narrow portion directed anteriorly; the posterior rounded. It is coarsely pitted medially, and coarsely and strongly radiate-ridged to the margin. The abdominal armature commences immediately behind the pectoral girdle. It consists of elongate, narrow, subcylindric scales, which meet on the median line, converging anteriorly. Small limbs are present. Measurements of Ptyonius marshii Cope. Type specimen: mm. Length of fragmentary skull 7 Width of skull as preserved 5 Length of entire specimen 68 Width across pectoral plates 8 Width across belly 7 Specimen No. 1098 G, American Museum: Length of specimen 32 Width of specimen 7 4 scutes of abdominal armature in 1 mm. =Ptyonius nummifer Cope.= Cope, Geol. Surv. Ohio, II, pt. II, pp. 374, 375, pl. xli, figs. 2 and 3, 1875. Moodie, Bull. Am. Mus. Nat. Hist., XXVI, p. 356, pl. 63, fig. 3, 1909.