_Var._ (11) d'Orbignii (_Chenu_) (Pl. 1, fig. _l_; Pl. 2, 1 _m_, 1 _n_): _radii with their summits oblique, and the orifice of the shell rather deeply toothed; shell conical or tubulo-conical, smooth, or rugged; colour dull purplish-lilac, with the tips of the parietes and a band along one side of the radii quite white; sheath rather darker at the rostral than at the carinal end; scuta as in_ var. 1; _terga as in_ var. occator. _Habitat._--West coast of Africa, on mytili; Madeira, on rocks; West Indies; Cape of Good Hope, on a patella and on kelp; mouth of the Indus; East Indian Archipelago; Sydney, Australia, attached to Lepas anserifera, adhering to a floating cane; Peru; Galapagos Islands; West Mexico; California. Extremely common on ships' bottoms arriving from West Africa, India, and China, often associated with _B. amphitrite_. _Fossil_ Red Crag, England; Mus. S. Wood and J. de C. Sowerby.--Touraine (?); Mus. Lyell. _General Remarks._--This, the first species of Balanus, is, perhaps with the exception of _B. amphitrite_, the most difficult and variable in the genus. There are some other species which vary quite as much in external appearance; for instance, _B. perforatus_; but _B. tintinnabulum_ also varies in far more important points, as in the proportions and structure of the opercular valves. The difficulty in determining whether or not the differences are specific, is wonderfully increased by whole groups of individuals varying in exactly the same manner. I have seen three most distinct varieties taken from the bottom of the same vessel, so that I did not at first entertain the least doubt that they were three distinct species. I may mention, as showing the vacillations which I have experienced on this subject, that beginning with the impression, that the above three varieties were really distinct species, after going over the several immense collections of specimens placed at my disposal, I came to the conclusion that the above three, and several other forms presently to be described, were only varieties; yet after an interval of some months, having to look at some of these specimens again, I could not but think that I had come to a false conclusion, and so went into all the details again, and satisfied myself that I had followed a right course; after another interval, I had to repeat the same process, and even now I can never look at a group of the beautifully coloured shells with their small rounded orifices of _var. coccopoma_ attached to the _Avicula margaritifera_, or again at _var. d'Orbignii_, with its toothed orifice and white tips to the compartments, without thinking that they must be specifically distinct from the dull-coloured specimens with large entire orifices so common on ships' bottoms; yet I can produce a full series of intermediate forms, and I can further show, in each variety, that