I have had occasion lately to thoroughly investigate these and other allied Ammonites, partly because it has been important to me that I should know the true affinities of these species, partly because my attention was directed to certain still obscure points with regard to their identification, and partly on account of the statement by the late Dr. Wright that Am. serpentinus and Am. falcifer were the same species. In pursuing my investigations I have received a great deal of assistance from Dr. E. Hang's Beiträge zu einer Monographie der Ammoniten-gattung Harpoceras, which I am pleased to acknowledge, although I do not find myself able to agree with him in one or two small points which I will presently mention. Meanwhile, by the aid of a few references to well-known works, I will indicate the Ammonites so that they may be understood.