Hyaenodontidae of the upper Eocene of California

Since the announcement(1) of the discovery of upper Eocene mammals in the Sespe deposits north of the Simi Valley, California, excavations have been continued in these beds, particularly at Locality 150 C. I. T. Vert. Pale., by field parties of the California Institute. Recently an opportunity has been afforded the writer to compare a number of the mammalian types found here with related forms known from the late Eocene and Oligocene. This has permitted a more precise determination of the genera and species than was attempted in the original statement. It is planned therefore to present in this and in subsequent papers brief descriptions of individual types, reserving a fuller discussion for a monographic treatment of the Sespe faunas.