I am greatly indebted for assistance to several individuals and institutions. In addition to the collection of specimens in the Museum of Zoology of the University of Michigan, which served as a nucleus for the work, I have had access to the collections of many museums and universities and to the private collections of several individuals. Often single specimens proved of unusual value; some by showing an extreme variation, others by contributing a new locality record. Every additional specimen, even though it possess no unusual features, is an important contribution in swelling the numbers necessary for accurate interpretation of individual, sexual, and geographic variation.