LEPAS PURPURASCENS.[110] _Wood's_ General Conchology, p. 55, Pl. 9,
fig. 42, 1815.
BALANUS PLICATUS. _Lamarck._ Animaux sans Vertèbres, 1818.
---- ---- ET PUNCTURATUS. _Chenu._ Illust. Conch., Tab. 4, fig. 3 et
12.
CONIA DEPRESSA (!). _J. E. Gray._ Appendix, Dieffenbach's Travels in
New Zealand, 1843 (sine descript. vel figurâ).
[110] The descriptions given by Wood and Lamarck are fuller and
more accurate than is usual in the case of Cirripedes, and I have
no doubt regarding these two names. The _Conia depressa_ of Dr.
J. E. Gray is, as I know from having seen the original specimens,
the young of this same species; the name is unaccompanied by any
description or figure.
_Shell depressed, pale purple or dirty white, with the surface
longitudinally ribbed, or corroded and granulated: radii or even
sutures none, or radii well developed and broad, with their summits
parallel to the basis: basis membranous: scutum transversely elongated:
tergum small, with the spur extremely short and rounded._
_Hab._--Sydney, New South Wales; Flinder's Lagoon, Sir C. Hardy's
Island, Barrier Reef; King George's Sound, Western Australia; Van
Diemen's Land; New Zealand, adhering to _Pollicipes spinosus_;
Mus. Brit., Cuming, Stutchbury, Darwin, &c. China (?) attached to
_Pollicipes mitella_, Mus. Brit. and Stutchbury. Generally attached
to tidal rocks, sometimes to shells. Very common.
_General Appearance._--Shell generally much depressed, in a few
cases rather steeply conical, in one single instance cylindrical,
but not much elongated. Colour, when alive, pale, but fine purple; I
presume, judging from some dried specimens, sometimes dirty white.
The state of the surface varies remarkably: about half the specimens
(fig. 1 _a_) which I have seen, had the outer lamina of shell quite
removed, and the surface granulated, owing to the projecting and
exposed tips of the upfilled parietal tubes; the radii are not
developed, and often even there is no trace of the four sutures;
the rather large orifice is somewhat rounded, and the two scuta,
with their surfaces disintegrated, have their middle parts deeply
indenting the terga. The shell, in the other and perhaps more common
condition (fig. 1 _b_), has the outer lamina preserved, and is
longitudinally ribbed with generally at least five or six ribs on
each compartment: the radii are here very wide, and extend from tip
to tip of the compartments, so that their summits are parallel to the
basis; they are generally covered by a brownish epidermis, thickly
clothed with little spines; the orifice is neatly diamond-shaped;
the apices of the opercular valves meet at a common point: these
specimens are almost always smaller and younger than the granulated
specimens. Altogether the specimens in the two opposite states have,
in their external appearance, nothing in common, and no one, without
careful examination, would ever suspect that they were specifically
identical; this, however, was proved by the intermediate forms,
and, in one instance, three of the compartments had their surfaces
granulated, and were entirely destitute of the radii, whilst the
fourth by some chance had been preserved from corrosion, was
longitudinally ribbed, and had its epidermis-covered radius fully
developed. The difference in the appearance of the opercular valves,
in the two states, is simply owing to the degradation of their upper
parts in the granulated specimens.
The basal diameter of the largest specimen was one inch, but the
height only .35 of an inch.
_Scuta_, transversely elongated, so that the basal margin is nearly
twice as long as the tergal margin: articular ridge very little
prominent; articular furrow wide but shallow; adductor ridge very
blunt, slightly prominent, sometimes almost absent, almost parallel
to the basal margin: there are no distinct crests for the rostral
or lateral depressor muscles, but some small irregular pits for the
latter. In one young specimen, the lines of growth were crenated,
showing a tendency in the valve to become longitudinally striated, as
in the allied _T. costata_. In some young and immature specimens, the
basal margin was deeply sinuous.
_Terga_, small in area, not above half that of the scuta: spur
extremely short, broad, placed close to the basi-scutal angle of the
valve, so that there is no basal margin on that side of the spur. The
lower end and sides of the spur form one uniform curve. Articular
ridge barely developed. Crests for the tergal depressores sharp and
prominent.
_Structure of the Shell and Radii._--The walls are very thick, and
the parietal tubes small and numerous; there are sometimes from
twelve to fifteen rows of tubes in the thickness of the wall. The
tubes in their whole upper part are filled up solidly; and, as
we have seen, are often exposed by disintegration. In very young
specimens, of that size in which in _T. porosa_ there would be only
a single row of parietal tubes, there were here two or three rows.
The development of the radii, as we have seen, is very capricious;
the sutures even sometimes being lost. The radii, when developed,
are broad, square on the summit, and covered by brownish hirsute
epidermis: internally they are formed of tubes like those forming the
parietes; in this respect differing from all the species except the
following, _T. costata_. The tubes in the radii run obliquely down
towards the basis; instead of in a transverse line, directly towards
the opposite compartment, as might have been expected from the
structure of the radii in Balanus. The alæ have their edges finely
crenated. The sheath in all the specimens which I have observed is
colourless; its lower edge is not free. The corium entering the
parietal tubes, and lining the opercular valves, the mouth, and the
anterior cirri, is generally of an extremely dark purple colour.
The _Basis_ is entirely membranous, in which respect this species
differs from all the others in the genus.
_Mouth_: all the trophi are unusually hairy or spinose. The labrum is
deeply notched and apparently destitute of teeth on the crest. The
palpi are club-shaped or enlarged at their extremities. The mandibles
have the fourth tooth rudimentary.
In the _Cirri_, the second and third pairs are remarkably short
and blunt. In one specimen the two rami of the first cirrus had
respectively six and sixteen segments; those of the second, six and
seven; those of the third, seven and seven; and those of the sixth
cirrus twenty rather elongated segments, with a small tuft of spines
between each main pair of spines.
_Affinities._--This species differs from all, in its membranous
basis, and in its transversely elongated scuta. It resembles _T.
costata_, and differs from all the other species, in its radii
(when developed) being square on the summit, and in being formed of
tubes,--in the smallness and number of the parietal tubes,--in the
absence of crests on the under side of the scuta for the rostral
and lateral depressor muscles,--in the shortness and rounded form
of the spur to the terga,--and, lastly, in the club-shaped palpi
and small size or absence of the fourth tooth in the mandibles. _T.
purpurascens_ differs from _T. costata_ in those points, namely, in
its membranous basis and transversely elongated scuta, in which it
differs from all the other species, and, moreover, in its scuta not
being plainly striated longitudinally, in having more ribs on the
external surface of the parietes of its shell, and in having only
three pairs of main spines on the three posterior cirri.