_Range: Geological History._--This species is common on the shores of Scotland and Ireland; the most southern point of Europe whence I have happened to see a specimen is Tenby, in South Wales: but I have no doubt it is found further south; and Mr. Jeffreys, who knows this species well, has found it common on the extreme southern shores of England. In the United States, it is found on the shores of Maine and Massachussetts: northward, I have seen specimens from Iceland, from Davis's Straits, and from Lancaster Sound, in lat. 74° 48′ north; these latter I owe to Sir J. Richardson. It is an inhabitant of deep water; in Mr. Thompson's collection there are several specimens from the Bay of Belfast, marked twenty-five fathoms, and one group said to have come from "about fifty fathoms, on the coast of Antrim:" one specimen from Cape St. Anne, Massachussetts, is marked as having come from only five fathoms. This species is commonly associated, on both sides of the Atlantic, with _B. crenatus_, and sometimes with _B. Hameri_ and _Verruca Strömia_: mollusca, such as pectens, modioli, and oysters, offer the most usual surfaces of attachment: I have, however, seen many specimens on crustaceans, on rocks, and even on the roots of the larger sea-weeds. This species is very common in the glacial deposits of Uddevalla, of Skien in Norway, and of Canada, and is associated with the same species as in the living state: I have seen, also, specimens from the same formation in the Island of Bute, Scotland. I have seen numerous specimens from the mammaliferous crag, and a few from the Red Crag of England. I owe to the kindness of Mr. J. de C. Sowerby an inspection of the original specimens of the _B. tesselatus_ of the Mineral Conchology, which is certainly the present species. _Affinities._--This species is very distinct from every other; it comes nearest, as shown in all the characters derived from its opercular valves, to _B. nubilus_, and in this latter species we have seen the basis plainly tending to lose its pores and thus become solid. _B. porcatus_ is perhaps allied in some degree to _B. trigonus_, and slightly to _B. crenatus_. The rather broad radii, with their summits hardly oblique, give this species a very different aspect from those species of the genus amongst which it must be placed.