_Shell depressed, whitish, generally with ten very prominent
longitudinal ribs: radii broad, with their summits parallel to the
basis: basis calcareous: scutum externally striated longitudinally:
tergum with the spur short and rounded._
_Hab._--Philippine Archipelago, Mus. Cuming. Attached to various
shells, within the tidal limit.
_General Appearance._--Shell whitish, probably tinged, when alive,
with reddish-purple; depressed; surface perfectly preserved, smooth,
but having longitudinal very prominent ribs, almost invariably ten
in number; namely, three on both the rostrum and carina, and two on
the two lateral compartments, with ten corresponding projections
round the basal margin. Orifice passing from rounded-trigonal to
diamond-shaped. The radii are very broad and square at the summit,
and extend from tip to tip of the compartments. Basal diameter of
largest specimen under half an inch, generally from .3 to .4 of an
inch.
_Scuta_, of the usual sub-triangular shape, and not transversely
elongated, as in _T. purpurascens_. External surface striated
longitudinally; in many specimens there is a medial depression, or
a row of very small pits, such as occur on the scuta of _Balanus
trigonus_ and _lævis_. The adductor ridge is moderately developed,
and runs nearly parallel to the occludent margin; there are no crests
for the rostral and lateral depressor muscles.
_Terga_: these in area equal two thirds of the scuta: the spur is
short and rounded, and placed as described under _T. purpurascens_;
but the articular ridge seems to be more prominent than in that
species.
_Structure of the Shell and Radii._--The parietal tubes are small,
and very numerous, as in _T. purpurascens_. The radii are wide,
square on the summits, but not so conspicuously covered by hirsute
epidermis as in that species. Internally, the tubes forming the radii
are smaller, and run more transversely than in _T. purpurascens_,
that is in the normal course, as in Balanus. The edges of the alæ are
nearly or quite smooth. The _Basis_ is as distinctly calcareous, as
in the other species of the genus.
_Mouth_: the trophi are not so hairy as in _T. purpurascens_; the
labrum seems destitute of teeth; the palpi are club-shaped at their
ends; the mandibles have only three teeth. The second and third
cirri are not so short and blunt relatively to the others as in _T.
purpurascens_. In the posterior cirri, the elongated segments carry
four main pairs of spines, between which there is no intermediate
tuft of fine spines.
The _Affinities_ of this species have been fully pointed out under
the last and closely related species. In external appearance, _T.
costata_ can at first hardly be distinguished from those young and
pale-coloured varieties of _T. purpurascens_, which have their
external surface not corroded, and their radii well developed.