_Erierpeton branchialis_ Moodie. Mazon Creek, Illinois, shales. Based on impression of mandibles and branchial apparatus. _Hyphasma lævis_ Cope. Linton, Ohio, Coal Measures. Based on incomplete and obscure amphibian body, lacking limbs. =Genus COCYTINUS Cope, 1871.= Cope, Proc. Am. Phil. Soc., 1871, 177. Cope, Geol. Surv. Ohio, II, pt. II, 360, 1875. Type: Cocytinus gyrinoides Cope. Vertebræ and ribs osseous; teeth on the premaxillary bone, none on the maxillary; hyoid elements largely developed, an axialhyal with basihyal on each side, closely united with the corresponding ceratohyal, at the end of which is an element in the position of a stylohyal; hæmal or basibranchials 3, the anterior 2, each supporting i pleural branchihyal, and the third supporting one also, the first hæmal branchihyal on the inner side of the ceratohyal, approaching the median line, and with elongate pleural element. The reference by Cope of this genus to the Caudata is one of the most interesting facts connected with the Paleozoic Amphibia. He says: "The present genus is, then, to be referred to the neighborhood of _Amphiuma_ and _Protonopsis_, but forming the type of another family" (123). He regards the branchial apparatus as being more fish-like than that of any of the modern genera. It is possible that _Cocytinus gyrinoides_ was a larval branchiate and consequently aquatic form. It should be more fully compared with _Erierpeton branchialis_ from the Mazon Creek shales when better known, as well as with _Hyphasma lævis_ from the Linton locality. All three of these forms are included, provisionally, under the Cocytinidæ. =Cocytinus gyrinoides Cope.= Cope, Proc. Am. Phil. Soc., XII, p. 177, 1871. Cope, Trans. Am. Phil. Soc., p. 278, 1874. Cope, Geol. Surv. Ohio, II, pt. II, pp. 364-365, pl. xxxix, fig. 4, 1875. Type: Specimen No. 8613 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. [Illustration: Fig. 16. Obverse of _Cocytinus gyrinoides_ Cope, from the Coal Measures of Ohio. × 2. _pmx_, premaxillæ: _mx_, maxilla; _m_, mandible; _ah_, axialhyal; _h_, basal branchihyal; _ch_, ceratohyal; _bv_, hæmal branchihyal; _b, bII, bIII, bIIII_, pleural branchihyals.] Two specimens of this interesting amphibian are known, one of them fairly complete (No. 2564, Am. Mus. Nat. Hist.). The type specimen consists of the inferior bones of the cranium in a fairly complete state of preservation, with the muzzle and its teeth; also 8 anterior vertebræ, with their short recurved ribs. The condition of the hyal elements in the type specimen is as follows: the hæmal elements of the first branchial arch are partially concealed on both sides of the ceratohyal. An expanded truncate face for attachment to the axial element is visible on both sides, but the body of the bone is flat and presents the edge of the specimen.