=Genus ŒSTOCEPHALUS Cope, 1868.= Cope, Proc. Phila. Acad. Nat. Sci., 218, 1868. Cope, Trans. Amer. Phil. Soc., XIV, 16. Cope, Proc. Phila. Acad. Nat. Sci., 1868, 217. Cope, Proc. Amer. Phil. Soc., 1871, 41. Cope, Geol. Survey Ohio, II, pt. II, 380, 1875. Type: _Œstocephalus remex_ Cope. Form slender and snake-like; caudal vertebræ with elongated, dilated, sculptured neural and hæmal spines. Cranium lanceolate. Teeth numerous, of nearly equal size. No pectoral shields known; abdomen protected by very numerous bristle-like rods, which converge forwards. A pair of weak posterior limbs; branchihyal bones present. In the only well-preserved species the cranial bones exhibit no sculpture from the parietal region forward. The genus is not very distinct from Ptyonius, but it can not be united with that genus until more complete material is available. The species of the genus share with _Ceraterpeton_, _Urocordylus_ (334), and _Ptyonius_, as well as _Crossotelos_ (98), from the Permian of Oklahoma, the elongation, sculpture, and expansion of the neural and hæmal spines. There are but 2 species, which Cope distinguishes by the following characters: I. Vertebræ elongate; fan-like caudal processes narrowed. Size large; mandibular teeth of unequal lengths, with the apices turned backward _Œstocephalus remex_ II. Species only known from cranial bones with teeth. Teeth equal, erect, with acute conic apices, 11 in 5 mm _Œstocephalus rectidens_ =Œstocephalus remex Cope.= Cope, Proc. Phila. Acad. Nat. Sci., p. 217, 1868 (_Sauropleura remex_). Cope, Proc. Phila. Acad. Nat. Sci., p. 218, 1868 (_Œstocephalus amphiumianus_). Cope, Trans. Amer. Phil. Soc., XIV, p. 17. Cope, Geol. Surv. Ohio, vol. II, pt. II, p. 381, pl. xxvii, fig. 5; pl. xxxi, fig. 1; pl. xxxii, fig. 1; pl. xxxiii, fig. 2; pl. xxxiv. fig. 4, 1875. Moodie, Proc. U. S. Nat. Mus., 37, p. 27, 1909. Type: Specimen undetermined. The following specimens are to be found: Specimens of _Œstocephalus remex_ Cope in the National Museum, Nos. 4511, 4460, 44/8. There is one specimen of _Œstocephalus remex_ in the University of Chicago. Specimens of the species in the American Museum of Natural History: Nos. 121, 8322 G, 8694 G, no number, 8656 G, 8583 G, 8659 G, 19, 120, 8655 G, 8662 G, 8708 G, 8665 G, 112, 8663 G, 8581 G, 8658 G, 8660 G, 8700 G, 8469 G, 1102 G, 1152 G 142, 8381 G, and obverse, 21, 8664 G, 8672 G, 8592 G, 8684 G. Horizon and locality: Linton, Ohio, Coal Measures.