_Shell white: radii narrow, with their upper margins smooth, slightly
arched, very oblique. Tergum with a longitudinal furrow; spur with the
end rounded._
_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.