BALANUS EBURNEUS. _Aug. Gould_ (!). Report on the Invertebrata of
Massachussetts, 1841, fig. 6.
_Shell yellowish white. Scutum striated longitudinally: tergum with the
spur truncated, the basi-carinal margin generally much hollowed out,
and the carinal margin protuberant in the upper part._
_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.