Established on a species which presents its principal peculiarities in the structure of the vertebræ. Two specimens present inferior views of the spinal column, showing that the genus possesses, like _Siren_, enlarged diapophyses, but they are peculiar in their fan-like form. They resemble slightly the neural spines of the caudals of _Ptyonius_, but are present on the dorsal vertebræ. Whether the caudals of the present species possess ornamented neural spines the specimens do not indicate. The abdomen is protected by the usual hair-like rods arranged _en chevron_, the angle directed forwards. No indications of limbs can be discovered on the blocks. Without the cranial bones the affinities of this genus can not be determined; while it may be allied to _Cocytinus_, the vertebræ of that form are without peculiar diapophyses. =Thyrsidium fasciculare Cope.= Cope, Geol. Surv. Ohio, II, pt. II, p. 365, pl. xxxi, fig. 2, 1875. Type: Specimen No. 8552 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. The best preserved example of this species includes 9 vertebræ and the corresponding ventral armature. The centra, seen from below, are much contracted in their form, presenting an obtuse median rib, which expands to the articular extremities. In one or two instances the latter are divided by fracture, and the moderately concave form of the adjacent surfaces is displayed. The diapophyses are of complex form, but the details are concealed by the prevalent thin layer of coal which invests them. An inferior prominence runs parallel to the centrum; outside of this the process is obscurely trilobate and thickened, not flattened, as in the caudal vertebræ of _Ptyonius_. Several ribs (fig. 8) of moderate thickness appear by the side of the diapophyses. Eleven abdominal ribs in 5 mm. The second specimen was originally referred to _Œstocephalus remex_, as a posterior portion of its vertebral column, immediately preceding the caudal series. This reference appears to be incorrect, although the resemblance between the corresponding parts in the two genera is no doubt considerable, and the alternative of proposing a new genus and species was not at that time advisable. The neural spines are longer than high, and are nearly in contact at their margins; each is marked by about 5 obtuse vertical ribs. A fractured section of the abdominal spines in place displayed at least six layers of them. The material on which the above account is based is imperfect. The specimen figured by Cope (Geol. Surv. Ohio, vol. II, pt. II, pl. xxxi, fig. 2, 1875) is undoubtedly a portion of the vertebral column of _Œstocephalus remex_. Nos. 4462 and 4480 of the United States National Museum may be representatives of _Thyrsidium fasciculare_, but they are more probably _Œstocephalus remex_; if they are the latter this leaves the type as the only known specimen of the species. Measurements of Type of Thyrsidium fasciculare Cope.