Hab.—United States, from about lat. 42° to Charlestown; West Indies; Honduras; Venezuela; attached to shells and floating wood. Attached to ships’ bottoms from Trinidad and Jamaica, associated with [B. tintinnabulum], [amphitrite], and [improvisus]. Brackish water, Salem, Massachussetts, according to Mr. Stimpson. Mus. Aug. Gould, Agassiz, Stutchbury, Cuming, W. Dunker, &c.; very common.
General Appearance.—Shell conical, or almost tubular; white, with the surface very smooth, covered by thin yellowish epidermis, but with the radii naked. Orifice large, passing from rhomboidal into pentagonal, moderately toothed. Average full size, about one inch in basal diameter; I have seen a specimen 1.3 in basal diameter, and the same in height.
Scutum, plainly striated longitudinally: the teeth on the occludent margin small. Internally, the upper surface is roughened: the articular ridge is prominent, and either slightly or not at all reflexed: the pit for the adductor muscle is distinct; the adductor ridge is prominent in a variable degree, and is almost confluent with the articular ridge. In one specimen from Beverly Bay, U. S., the scuta were extraordinarily disintegrated, and I could perceive no trace of the external radiating striæ. Tergum, with the basal margin on the carinal side of the spur sometimes deeply (Pl. [5], fig. [4 b]), and sometimes only slightly (fig. [4 d]), and rarely hardly at all, hollowed out: when much hollowed out the valve may almost be said to be two-pronged, with the carinal prong narrower than the spur. There is no distinct longitudinal furrow, but the whole scutal margin projects above the general surface of the valve. In the carinal margin, in the upper part, there is a remarkable convexity or protuberance in the same plane with the valve, from which it is separated by a very slight and narrow ridge. The spur is about one fourth of the width of the valve, with its lower end abruptly truncated. Internally, the upper surface is much roughened with finely crenated ridges: the distinct crests for the depressores cover the whole of the so-called carinal prong.
Compartments; the radii and alæ have their summits oblique, sometimes a little rounded, but not smooth. The septa on the sutural edges of the radii are remarkably fine, and closely approximate; the denticuli are excessively minute. The sutural edges of the alæ are most delicately crenated; the alæ are largely added to during the diametric growth of the shell, and above the level of the opercular membrane. The parietal pores are square and rather large: they are crossed by transverse septa almost close down to the basis: the longitudinal septa have tolerably large denticuli at their bases. The pores in the basis are crossed by numerous transverse septa. When specimens grow in a group, the basis is sometimes irregularly cup-formed.
Mouth: labrum serrated with small teeth, decreasing in size downwards, on each side of the central notch. Mandibles with the third tooth rather thick and blunt, and with the fourth and fifth knob-like. Maxillæ, with the inferior part projecting much beyond the rest of the edge, and bearing two long single spines: between these two spines and the large upper pair, there are, in a full-sized specimen, about seven pairs of moderately long spines, feathered on their sides. Outer maxillæ thickly clothed with very fine spines, and remarkably prominent.
Cirri: first cirrus, with one ramus having twenty-six segments, and longer by ten segments than the shorter ramus, which has sixteen segments: the shorter ramus, and both ramii of the second pair, have their segments remarkably protuberant in front; the protuberance, in the upper segments, equalling in length the supporting part of each segment: rami of the second cirrus unequal in length by five segments. Third cirrus with the segments only slightly protuberant; rami considerably longer than those of the second cirrus: at the dorsal base of the pedicel of this third cirrus there is no tuft of fine hairs, as is common in many other species. Sixth pair, with the upper segments elongated, bearing from six to seven pairs of spines; dorsal spines short, thin, and few.
Affinities: in external appearance of the shell, this species can hardly be distinguished from some of the white varieties of [B. amphitrite]; and there is a considerable resemblance, in some of the varieties, in the opercular valves; but the longitudinally striated scuta of [B. eburneus] suffice to distinguish these certainly very distinct species. Equally, or even more like externally, is this species to the [B. Hameri], so that I have received from an eminent naturalist in the United States both species mingled in the same lot, all bearing the same name of [B. eburneus]; but when the internal structure of the shell is examined, the species are at once seen to be far removed from each other. Still more close is the affinity of this species to [B. improvisus], both in internal and external characters: it agrees with this species in the singular habit of being able to live in brackish water: these two species are the only ones which have the labrum serrated with teeth, graduated in size, on each side of the central notch. In the case of young specimens of the var. assimilis of [B. improvisus], an inhabitant of the same seas with [B. eburneus], the diagnosis is most difficult without long practice; for in the young of [eburneus], the compartments are only partially covered by yellow epidermis, and have a striped appearance, the radii are sometimes very oblique, the scuta externally have not acquired their longitudinal striæ, and internally the adductor ridge lies not so close to the articular ridge as it does subsequently; hence I for some time mistook the var. assimilis of [B. improvisus] for the young of [B. eburneus]. But I found in the latter, that the rami of the first pair of cirri, are always, even in the earliest youth, more unequal in length, and that each segment of the posterior cirri bears a greater number of pairs of spines, there being, even in very minute specimens, seven pairs. Moreover, after having examined scores of specimens, I found I could almost always distinguish the two species by the smoothness and curvature of the summits of the radii of [B. improvisus]; I entertain no doubt whatever about the distinctness of the two species; indeed, when both are mature, besides the greater size, striated scuta, &c. of [B. eburneus], their general aspect is very different.
22. [BALANUS] IMPROVISUS. Pl. [6], fig. [1 a]-[1 c].
Shell white: radii narrow, with their upper margins smooth, slightly arched, very oblique. Tergum with a longitudinal furrow; spur with the end rounded.