The pectoral arch is almost unknown, and Cope based the distinction of _Ptyonius_, and _Œstocephalus_ on the absence of these plates in the latter genus an uncertain characterization. The fore limbs are indicated by a humerus. There were possibly from 75 to 100 vertebræ in the entire column. The animal was exclusively adapted to life in the water and was, without doubt, an excellent swimmer. There are preserved in one specimen portions of what seem to be hyobranchial elements. Measurements of Œstocephalus remex Cope. mm. Length of entire caudal series 195 Width at ninth vertebra 21 Width at thirty-sixth vertebra 2 Length of four phalanges in place 14 Expanse of fan of proximal caudal 18.5 Length of ilium 11.5 Length of femur 11.5 Length of tibia 7 Length of mandibular dental series 24 Depth of mandible at middle 5 Nine teeth in 5 Length of longer teeth 2 Length of first hæmal branchial 6 Length of sixth vertebra from skull 7 Width of centrum 3 =Œstocephalus rectidens Cope.= Cope, Trans. Amer. Phil. Soc., p. 268, Apr., 1874. Cope, Geol. Surv. Ohio, II, pt. II, p. 386, pl. xxvii, fig. 3, 1875. Type: Specimen No. 9033, American Museum of Natural History, collection of J. S. Newberry. Horizon and locality: Linton, Ohio, Coal Measures. The species is indicated by a left dentary bone, with its teeth and external surface preserved. The latter is nearly smooth and without sculpture. The outer face is convex, and the general form is slender, but not curved upward at the extremity. The extremity of the dentary does not show any evidences of teeth. Teeth straight and conic, apex acute, non-plicated. Cope also associated with this species a portion of a caudal series, consisting of 25 vertebræ. The centra are elongate and expanded at the extremities. The neural arches have a close union. The neural and hæmal spines are fan-shaped and striated. The bases are quite narrow. Measurements of Œstocephalus rectidens Cope. mm. Length of dentary 22 Length of tooth line 15.2 Depth of dentary at last tooth 2.7 Length of 3 centra of the caudal series 8.6 Extent of neural and hæmal spines 8.7 =Genus THYRSIDIUM Cope, 1875.= Cope, Geol. Surv. Ohio, II, pt. II, p. 365, pl. xxxi, fig. 2, 1875. Type: _Thyrsidium fasciculare_ Cope.