Now if we reflect that form, size, state and nature of the surface, presence of epidermis, relative size of the orifice, presence of longitudinal ribs, tint, and often the existence of any colour, are all highly variable in most of the species; and that the obliquity of the summits of the radii, and the presence of longitudinal striæ on the scuta, are variable in several species; we shall perceive how difficult it must ever be to distinguish the species from external characters. As some evidence of this, I may mention that, after having described nearly 40 species, and when my eye was naturally able to appreciate small differences, I began carefully to examine varieties of _B. tintinnabulum_, _amphitrite_, _improvisus_, _porcatus_, _vestitus_, &c., without even a _suspicion_ that they belonged to these species, at that time thoroughly well known to me; yet in the cases here referred to, there could be no doubt, when a perfect series was examined, that the specimens were only varieties. From this cause the labour of naming a collection is great. Let no one attempt to identify the species of this genus, without being prepared to disarticulate, clean, and carefully examine with a microscope the basis and parietes, and both the under and upper surfaces of the opercular valves; for I feel convinced, that he will otherwise throw away much labour. Moreover, in many cases, it is almost necessary, on account of the variability of the characters, to possess several specimens. From these facts, I have not hesitated to form my sections on characters which require close examination, though I would gladly have seized on external characters, could I have found such even moderately constant.