_Geographical Distribution._--This species is extremely common over the whole of the warmer seas. It ranges from the Island of Madeira to the Cape of Good Hope, and on the west coast of America, from Monterey, in lat. 37° N., in California, to Peru. It is attached to rocks and sub-littoral shells, to floating timber, to kelp, and to _Lepas anatifera_. It is attached in wonderful numbers to ships' bottoms arriving at our ports, from West Africa, the West Indies, the East Indian Archipelago, and China. It is generally associated with _B. amphitrite_ and _amaryllis_. I have already stated that, on the bottoms of vessels, the different varieties are generally grouped together; and this makes me believe that they are local. In Mr. Stutchbury's collection there are numerous specimens taken from a ship which first went to the west coast of Africa for guano, and then to Patagonia for the same object, and it was interesting to see the manner in which numbers of _B. psittacus_, a Patagonian species, had become attached on the African _B. tintinnabulum_. The varieties from the west coast of America seem eminently peculiar; we there find _var. coccopoma_ and _concinnus_; and a blueish, rugged variety, with peculiar opercular valves. _Geological History._--I have seen specimens in Mr. S. Wood's collection from the Red Crag of England, which, though not accompanied by opercular valves, I cannot doubt belong to this species. The specimens named by Mr. Sowerby, in the 'Mineral Conchology,' as _B. crassus_, and which I have seen through the kindness of Mr. J. de C. Sowerby, also belong to this species. I further believe that a specimen in Sir C. Lyell's collection, given to him by M. Dujardin under the name of _B. fasciatus_ from Touraine, is likewise _B. tintinnabulum_. _Varieties._ With respect to _var. communis_, I have nothing to remark. The second, _var. vesiculosus_ (Pl. 2, fig. 1 _h_), is confined to young specimens, and may, perhaps, be due to a want of calcareous matter. With respect to _var. validus_ (Pl. 1, figs. 1 _c_, 1 _f infra_), I may observe that some of the coarsest and strongest specimens which I have seen were said to have been attached to a surface of iron. I have seen two large lots of _var. zebra_ (1 _g_), taken by Mr. Stutchbury, from the bottoms of ships, arriving from Bengal and China, and in both cases associated with _var. communis_, and in one case with _var. coccopoma_. I at first thought that this variety, _zebra_, was specifically distinct, but now I feel no doubt, that it is a mere variety; its body was in every respect identical with that of _var. communis_.