Every collection of western Bythoscopidæ that the author has Examined has had a more or less heterogeneous assemblage of forms closely resembling Idiocerus alternatus. These have been somewhat Indiscriminately divided between alternates, ramentosus and several MS names of Uhler‖s in classification, or not determined at all. At the time of publication of the key to this genus, the material at hand was not sufficient to warrant the descriptions of these forms, and they were omitted. Since then the author has paid special attention to the collecting of this, group in Colorado, and has received a long series of specimens from the Pacific Coast, mainly from the collection of Mr. C. Livingstone. With this material it has been possible to accurately define a number of these species and clear up much of the obscurity in this section of the genus. During the progress of this study several other new species and varieties have been described, and are also presented here.