The Compartments have their radii developed to a rather varying degree, with their summits oblique; hence the orifice is toothed: the sutural edges of the radii have their septa barely denticulated; the sutural edges of the alæ are smooth. The basis, as with the other species of this section, is permeated by pores; yet I found one specimen, from the Cape of Good Hope, with the basis apparently solid, thus offering a very singular anomaly. In the specimen imbedded in sponge, the basis, as viewed externally, is concave; whereas in [Acasta], which always inhabits sponges, the basis is highly convex or hemispherical.
The Mouth and Cirri resemble those of [B. trigonus], and I can point out no distinguishing character.
With respect to the variety from the West Indies, I have seen two sets of specimens differing somewhat in external appearance, one set attached to a coral from St. Vincent’s, and another set to an Avicula from an unknown locality; at first I described these specimens, with some hesitation, as a distinct species, and I am very far from sure whether this would not have been the more correct course, although I am unable to point out any sufficient diagnostic characters. This form differs from the ordinary [B. spongicola], in the walls being more rugged, stronger, and slightly or deeply folded longitudinally; in this latter case the shell in external aspect differs much from ordinary specimens of [B. spongicola]; but this is a variation so common that I dare not place any reliance on it. The colour is more purple; the summits of the radii perhaps rather less oblique. In the scuta the only difference is that the articular ridge seems rather longer, and the adductor ridge perhaps more prominent: in the terga, as already remarked, the basal margin on the carinal side does not slope so straight into the spur. These differences I consider all too slight to be of specific value. The difficulty in determining the nature of this variety is added to by its approach to [B. trigonus] in all those points in which it departs from the ordinary [B. spongicola], so that for a short time I was even tempted to consider both these species as varieties of one form. But until [B. trigonus] is found with its scutum longitudinally striated, and with its tergum beaked, it can hardly be confounded with [B. spongicola]; for it should be observed that when in [B. trigonus] the rows of little pits disappear from the scuta, as sometimes happens, though rarely, yet these valves do not become longitudinally striated.
[Balanus spongicola] occurs, mingled with [B. tulipiformis], in the Mediterranean, and by the external characters of the shell alone cannot be distinguished from that species; but the striated scuta and beaked terga suffice to separate them. Again, this species, at the Cape of Good Hope, occurs mingled with [B. Capensis], and from the non-striped young varieties of that species, it can, externally, be distinguished only by the beak of the tergum not being sharp like a needle. I have seen a single, perfectly characterised specimen, with its opercular valves preserved, found by Mr. S. Wood in the Coralline Crag at Sutton, mingled with [B. inclusus].
16. [BALANUS] LÆVIS. Pl. [4], fig. [2]-[2 g].
BALANUS LÆVIS. Bruguière. Encyclop. Meth. (1789), Pl. 164, fig. 1.[91]
------ DISCORS. Ranzani. Mem. di Storia Nat., 1820, Tab. 3, figs. 9 to 13.
------ COQUIMBENSIS. G. B. Sowerby, in Darwin’s Geology of South America (1846), Tab. 11, fig. 7.