_Mouth._--Labrum with three teeth close together on each side of the central notch: mandibles with four teeth, the fourth being small, the fifth either absent or scarcely distinguishable from the inferior angle: maxillæ without any notch, with the two lower spines rather longer than the others. _Cirri_: In the first pair, one ramus is only half the length of the other; in the second pair, both rami are short and about equal in length; in the posterior pairs, the segments, which are not protuberant, bear four pairs of spines, of which the three lower pairs are short. This species is widely-distributed, and where found seems to be common. It is generally attached to shells of mollusca, but I have seen it also attached to wood. I have found it associated with _B. tintinnabulum, var. concinnus_, and _coccopoma_, with _B. psittacus_, _improvisus_ and _amphitrite_, and with _Elminius modestus_. Young specimens bear a considerable resemblance to certain young varieties of _B. tintinnabulum_, and can indeed be distinguished from them only by a careful examination of the opercular valves; for it should be borne in mind, that in certain cases the scuta in _B. tintinnabulum_ are pitted with little cavities. This species in some respects is, I think, allied to _B. porcatus_, but it is far more closely related to _B. spongicola_, and can be discriminated with difficulty from certain varieties of this latter species. In Mr. Cuming's collection, there is a group of small specimens, crowded between some older specimens, which are remarkable from the shell being oval in a transverse section,--from the smoothness of the walls,--and from the absence of pits on the scuta; yet there could be no doubt that these specimens belonged to our present species. 15. BALANUS SPONGICOLA. Pl. 4, fig. 1 _a_-1 _d_. BALANUS SPONGICOLA. _Brown's_ Illustrations of the Conchology of Great Britain (1827), pl. 7, fig. 6: 2d edit. (1844), pl. 53, figs. 14-16. _Parietes generally smooth, sometimes longitudinally folded; coloured pink: orifice toothed. Scutum longitudinally striated. Tergum, with the apex produced, without a longitudinal furrow; spur truncated, about one third of width of valve._ _Var. with the walls slightly folded longitudinally._ _Hab._--South coast of England, and Tenby in South Wales, often imbedded in sponges; attached also to shells and rocks in deep water; Mus. Brit., Jeffreys. Algiers, on Mytili and Serpulæ, with _B. perforatus_, Mus. Mac Andrew. Madeira, with _B. tulipiformis_, Mus. Lowe. Lagulhas Bank, Cape of Good Hope, on detached kelp, with _B. Capensis_, Mus. Sir J. Ross. Imbedded in sponge with _Acasta spongites_, Mus. Bowerbank. _Var._ West Indies. _Fossil_ in Coralline Crag, Mus. S. Wood.