_Shell pale purple, or white, or dirty ash-colour; smooth, or, from being corroded, finely ribbed longitudinally; sheath purple; orifice generally small; radii generally narrow or absent. Scutum, internally, with a short minute ridge, parallel and close under the prominent adductor ridge. Tergum with the apex somewhat produced._ _Var._ angustus (_Gmelin_) Pl. 5, fig. 1 _a_: _pale dull purple or white; orifice small or of moderate size; radii very narrow or moderately wide, white or pale purple, with oblique summits_. _Var._ Cranchii (_Leach_) Pl. 5, fig. 1 _b_: _corroded, covered with fine longitudinal ridges owing to the exposed, filled-up, parietal tubes; dark dirty ash-colour, with a tinge of purple: radii not developed, or very narrow with oblique summits; orifice small_. _Var._ fistulosus (_Poli_) Pl. 5, fig. 1 _d_: _shell cylindrical, white or dull purple; orifice of moderate size or small; basis deeply cup-formed_. _Var._ mirabilis, Pl. 5, fig. 1 _c_: _bright purple; radii white, very broad, with their summits parallel to the basis; orifice entire, large_. _Hab._--Southern shores of England; South Wales; Mediterranean; Western Africa, southward to Loanda, in 9° S.; West Indies (?). Generally adhering to rocks at a low tidal level; in one case attached to the floating _Lepas Hillii_, Mus. Jeffreys. This is a well-marked species, and in its essential characters does not vary much; but owing to the shell being almost as often white as purple,--to its being remarkably subject to disintegration,--to its often becoming cylindrical,--to the radii being either not at all, or slightly, or moderately, or largely developed, and consequently to the orifice of the shell varying in size, the general external appearance of the different varieties is singularly diversified; but when a series of specimens is examined, it is easy to see how one form passes into another.