This species is one of the most abundant of the Linton Amphibia. Cope based his description of the species on 9 specimens. There are more than two dozen available at the present time, the majority of them being in the possession of the American Museum of Natural History. There is a single specimen in Walker Museum of the University of Chicago and 3 in the United States National Museum. The numbers of all of these specimens are given above. The material consists, for the most part, of fragmentary portions of the vertebral column, but there are a few skulls more or less complete, though none are sufficiently well preserved for a complete analysis of the characters. The specimens indicate an animal slightly smaller than the modern _Amphiuma_ means of the Mississippi River. [Illustration: Fig. 31. Restoration of _Œstocephalus_. × 1.] It will not be necessary here to enter into a detailed account of each specimen, since this has been done by Cope, and a careful comparison of his descriptions with the originals indicates that his observations are correct. The species, as suggested in the discussion of the genus, is not clearly distinct from those of _Ptyonius_, and it has largely the characters of that genus. The cranium is long, slender, and wedge-shaped. The teeth are numerous both in the maxillary and in the mandible, one specimen indicating about 30 in a single series. They are all uniformly cylindrical, except at the extremity, where they are flattened and expanded so as to produce a longitudinal edge, which is carried backward on a recurvature of the apex. The bases are anchylosed equally and without enlargement, and no part of the shaft is striate or grooved. The upper surface of the cranium is narrow, with the median suture distinct. The skull surface, with that of the mandible, is smooth. Characteristic of the species are the remarkable length and slenderness of the fan-shaped neural and hæmal spines, and the absence of an acute serration on their margins. In this species the spines have a laminiform expansion at the base in their plane. One specimen exhibits the pelvic region, including a portion of the tail. The ilium has an expanded anterior extremity and is directed backwards and somewhat inwards on either side of the vertebral column. The femur is nearly straight, short, contracted medially, and expanded distally. The tibia is shorter and is subcylindrical. Beneath the ilium the last chevron of the abdominal rods appears, the outer extremities rising on the base of the tail.