Var. assimilis, with longitudinal white hyaline lines.
Hab.—England, Scotland, Belgium (?), Nova Scotia, United States, West Indies, Rio Plata, Southern Patagonia, Guayaquil, West Colombia; attached to wood, shells, rocks, ships’ bottoms, from low tidal level to twenty fathoms depth.
General Appearance.—Shell conical, with a rather large diamond-shaped orifice, moderately or but little toothed; very smooth; walls never folded longitudinally; white, with an extremely thin pale-yellow persistent epidermis. The radii are very narrow, with their summits very oblique, rounded, and smooth; the epidermis is generally more persistent on the radii than on other parts, and this is exactly the reverse of what is common with [B. eburneus]. The specimens from nearly fresh-water in the R. Plata (hereafter to be mentioned), are brownish, and have undergone a remarkable degree of corrosion, the outer lamina of the walls having been entirely removed to near the base; hence the external aspect of these specimens is wholly different from ordinary individuals. The var. assimilis has also a very different appearance, owing to the dead white of the walls being relieved by narrow approximate longitudinal hyaline lines, corresponding with and caused by the longitudinal parietal septa being externally visible through the outer lamina of the parietes; the epidermis on the radii is also of a rather brighter yellow. The largest specimens which I have seen are those from the Plata, and those attached to a ship from the West Indies, and they had a basal diameter of .6 of an inch: from .4 to .5 of an inch is the more usual full average size.
Scuta, with the lines of growth but little prominent: articular ridge prominent, but little reflexed: adductor ridge straight and very prominent, varying a little in its distance from the articular ridge; there is scarcely any depression for the lateral depressor muscle; the upper internal surface of the valve is roughened with ridges. Terga, with a moderately deep longitudinal furrow; spur short, rather narrow, with the end rounded, placed at less than its own width from the basi-scutal angle; in the Rio Plata specimens the spur is close to this angle: the basal margin is generally straight on opposite sides of the spur, but sometimes on the carinal side it is a little hollowed out. The lines of growth are upturned along the carinal margin, which consequently is a little protuberant, but to a varying degree. The crests for the depressores are extremely distinct and prominent. In the varieties having the basi-carinal margin hollowed out, and the carinal margin protuberant, there is a marked resemblance to the peculiar tergum of [B. eburneus].
Walls: the parietal pores are tolerably large, and are crossed by numerous transverse septa: the longitudinal septa are very finely denticulated at their bases, but occasionally almost smooth. The radii are, as stated, extremely narrow, and very remarkable from their smooth rounded edges; their septa are barely denticulated. The alæ are remarkably protuberant; they have their summits much less oblique than those of the radii, and sometimes they are almost parallel to the basis: their sutural edges are coarsely crenated. Basis, flat, thin, permeated by pores, but the pores do not generally run to the very centre; they are, as usual, crossed by transverse septa.
Mouth: the labrum is the most remarkable part; on each side of the central notch there are generally two teeth; and on the two sides of the notch itself nine or eleven smaller teeth, decreasing regularly in size downwards till they become so minute as to be hardly visible even under the compound microscope; thus, in the two specimens closely examined, there were altogether twenty-two and twenty-six teeth on the labrum. Mandibles with the two inferior teeth reduced to mere knobs: maxillæ with the lower part of the edge bearing two large spines, and generally, but not always, forming a step-formed projection. Cirri: the ramii of the first pair are but slightly unequal; in one specimen examined there were fifteen segments in one ramus and twelve in the other: segments very protuberant in front. Second cirrus with the segments only slightly protuberant; segments thirteen. Third cirrus longer than the second pair, with the rami rather unequal in length: there is a tuft of long spines on the basal segment of the pedicel of this cirrus. Fourth cirrus twenty-two segments. Sixth cirrus, in the same individual, thirty-four segments: on each of these segments there are five or six pairs of spines. I may specify that the longer ramus of the first cirrus of a large Rio Plata specimen had twenty-four segments.
Varieties, affinities.—When I first met with the var. assimilis, misguided by its general aspect, I did not doubt that it was specifically distinct; I was strengthened in this view by the absolute identity of several hundred specimens attached to two vessels from Jamaica and Trinidad, in the West Indies, with one specimen from Charlestown, in the United States, sent me by Prof. Agassiz, and with several in three lots from the western tropical shores of South America: yet on close examination I can point out no one distinguishing character, either in the shell or animal’s body, excepting the longitudinal hyaline lines on parietes, due to the septa being externally visible. The presence of similar lines is variable in white vars. of [B. amaryllis] and [amphitrite], and they are seen in very young specimens of [B. eburneus]: hence it is impossible to consider so trifling a character as specific; moreover, lately I have seen a British specimen with hyaline lines, and some few other specimens in an intermediate condition. Under the head of [B. eburneus], I have stated that although that species and [B. improvisus], which in the West Indies are associated together, are most readily discriminated when old, yet when young, they so closely resemble each other that the eye requires much practice to separate them. On account of this species and [B. crenatus] being sometimes associated together on the shores of England, I have pointed out under [B. crenatus], the relative diagnostic characters of the two. The chief affinity of [B. improvisus] is certainly towards [B. eburneus]; but in the narrow, oblique, rounded, and smooth-edged radii, there is a relationship shown to the species in the last section of the genus, such as [B. amaryllis], and more especially to the fossil [B. dolosus]: so close is the resemblance in the external appearance of the shell, and in the structure of the opercular valves, to the latter species, that I for some time did not discover their distinctness. [Balanus improvisus] has hitherto been overlooked by naturalists, and has probably been confounded with [B. crenatus] or [balanoides].
Range and habits.—This species, as far as my experience goes, is commoner on the shores of Kent than on other parts of England: the first specimens which I met with, I owed to the kindness of Mr. Metcalf, they were attached to wooden stakes from Herne Bay, together with a single specimen of [B. crenatus]: I have seen other specimens from near Woolwich, from the Kentish oyster-beds, from Sandwich, and from Ramsgate. The only other British specimens which I have seen are from the River Itchen, in Hampshire, and from Loch Shieldaig, in Ross-shire (Mus. Jeffreys), from a depth of twenty fathoms. This species is often attached to wood. At Ramsgate, the specimens were attached to a small coasting vessel, and they must have been immersed five or six feet; they were associated with [B. crenatus], and with a few of [B. balanoides]. In the Brit. Mus. there are specimens collected by Mr. Redman, from Nova Scotia, in North America. When her Majesty’s ship Beagle was beached at Santa Cruz, in Southern Patagonia, numerous specimens were found adhering to her copper bottom, some so small as to show that the species breeds in those latitudes. Near Monte Video, in the estuary of La Plata, I found many large, but much corroded specimens, adhering to some rocks in a small running stream of perfectly fresh water. The rise of the tide is here small, but at high water the specimens apparently were for a short time covered by the waters of the estuary, here itself only brackish, and occasionally almost fresh. I took home some specimens, and placing them in perfectly fresh water they continued for many hours expanding and retracting their cirri with perfect regularity and vigour. Here then we have a [Balanus] capable of living in fresh water, and likewise in the saltest seas: even brackish water is a deadly poison to several, probably to most, species of the genus; but this, as we have seen, is not the case with the allied [B. eburneus]. The water, I may add, at Woolwich, on the Thames, whence I have received [B. improvisus], must at times be very brackish. I have already incidentally mentioned that the var. assimilis was attached in great numbers, associated with [B. eburneus], [tintinnabulum], and [amphitrite], on vessels from the West Indies: one specimen sent me by Prof. Agassiz, from Charlestown, was attached to a specimen of [B. eburneus]; and, lastly, I have seen three sets of the same identical variety attached to shells from Guayaquil (in Mus. Brit. and Cuming), and from West Colombia. Here, then, we have the same species with an enormous range, from Nova Scotia and Great Britain to South Patagonia; and, which is the case with scarcely a single mollusc, it lives both on the eastern and western tropical shores of the South American continent.