CORONULA PATULA. _Ranzani._ Memoire di Storia Naturale (1820), Tab.
3, fig. 25-28.
ASTROLEPAS LÆVIS. _J. E. Gray_ (!). Annals of Philosophy (new
series), vol. 10, 1825.
VERRUCA CANCRI AMERICANI. _Ellis._ Phil. Trans., vol. 50, 1758, Pl.
34, fig. 13.
_Shell steeply conical, very smooth and light; orifice large, generally
exceeding half the basal diameter of the shell: radii broad, smooth,
only slightly depressed._
_Hab._--Mediterranean; Gambia, West Africa; Charlestown; Jamaica;
Honduras; Brazil; Australia. Attached to Crustacea, smooth univalve
shells, and apparently to ships' bottoms.
_General Appearance._--Shell white, very smooth, of little specific
gravity, steeply conical, but not high; orifice broadly oval,
polygonal, very large, namely, generally exceeding half the basal
diameter of the shell. The summits of the compartments are usually
perfectly preserved, pointed, and often a little recurved. The radii
are rather broad, very smooth, with their summits slightly oblique
and arched: they are seated only a little below the general level
of the parietes. I have seen one specimen rather more than one and
a half inch in basal diameter, but this is an unusual size; this
species not growing to so large a size as the two foregoing forms.
_Structure of Shell and Radii._--The walls are here thinner than in
the two foregoing species; and the basal surface of a compartment
rarely equals half the basal diameter of the internal cavity of the
shell, measured transversely to its longer axis. The radiating septa
are also much thinner, generally sinuous, and so finely dentated
along their basal edges, that the teeth can be clearly perceived
only by the aid of a lens. The interspaces between the septa run up
to nearly the summits of the compartments, and hence the lightness
of the whole shell. The inner lamina of the parietes is here not so
thick (fig. 4), and is more distinct from the descending sheath than
in the foregoing species. The sheath is thin, like the radiating
septa; the medial loophole in each compartment, for the entrance of
a filament of corium, is much wider than in _C. testudinaria_, for
it is generally as wide as the bordering plate on either hand; and
in not a few specimens, the medial loophole is so wide as hardly
any longer to deserve being so called, for the descending sheath
is reduced to mere flattened pillars or legs on the sides of the
sutures. Although the parietes are here not nearly so thick as in
the two foregoing species, yet as the radii stand but little beneath
the level of the parietes, the outer lamina of the radius has a
considerable thickness, and is sometimes separated from the inner
portion of the radius by an interval, in the same manner as in _C.
testudinaria_; and the growing edges of the outer lamina exhibit
traces of the same mamillated structure as figured in that species.
The _Opercular Valves_ are apt to be rather narrower and more
elongated than in the foregoing species, with the occludent margin
of the scutum generally, but not always, only slightly sinuous:
externally the scutum is sometimes feebly striated longitudinally. In
the tergum (Pl. 14, fig. 3 _b_) the longitudinal furrow is generally
plainer, from being more distant from the carinal margin, and the
rudimentary spur itself is more prominent.
The mandibles usually have five narrow teeth; but I have seen
one specimen having only four teeth, and with the inferior angle
truncated. The cirri present no particular character.