†Notogoneus osculus Cope, the type species for the geographically widespread Upper Cretaceous to Upper Oligocene genus †Notogoneus (Gonorynchidae), is redescribed in detail based on much new material. This species is known from the Early Eocene Fossil Butte Member of the Green River Formation, southwestern Wyoming. It is geologically the youngest known species of the family Gonorynchidae in North America, although the family is known from Oligocene and Miocene lacustrine deposits of Europe and Australia and is extant today in tropical marine environments of the Pacific (Gonorynchus spp.).

In this paper we also correct a number of mistakes in previously published morphological descriptions of †N. osculus (e.g., in skull and caudal skeleton) based on a large, new sample of well-preserved, well-prepared specimens. Accurate description of this species is important because it is the type for the genus and it is also by far the best preserved of all known species of †Notogoneus. In addition, it may shed new light on the interrelationships of Gonorynchidae (work in progress by the authors).

†Notogoneus appears to have inhabited a freshwater, subtropical environment. Based on distribution data and species diversity within the Green River Formation, we suspect that †N. osculus may have been a migratory fish, living in the central (off-shore) regions of Fossil Lake during part of its life, and in connecting streams and rivers during other parts of its life. Most or all species of the genus †Notogoneus are known from freshwater deposits and species are known from North America, Europe, Asia and Australia. Species of the extant gonorynchid genus Gonorynchus are all marine and are known from the Indo-Pacific and southern Atlantic.