_Shell dull blueish-purple, with six small, membrane-covered apertures
between the sutures, close above the basis: tergum with the articular
ridge very short and prominent; spur very broad and rounded._
_Hab._--Sumatra; Philippine Archipelago; imbedded in the bark of an
Isis; Mus. Cuming, Stutchbury, Brit.
This species is perfectly distinct from the others, as shown by its
general appearance, its habits, and the structure of its opercular
valves: it is allied to _A. fenestrata_, in having membrane-covered
apertures between the compartments, and in some respects in its
opercular valves: it is also allied to _A. sulcata_ and _cyathus_ in
the parietes being often internally ribbed, in the basal cup having a
crenated edge, and in the anterior ramus of the fourth cirrus being
furnished with the minute hook-like spines.
_General Appearance._--Sub-globular, slightly compressed, with a
rather small orifice; smooth, but sometimes furnished with sharp
shelly points; dull purple, more or less dark, with the upper parts
of the walls often white. The radii are rather narrow, and generally
white, with their summits only slightly oblique, but variable in
this latter respect. The parietes of the carino-lateral compartments
are narrow, being only one sixth of the width of the parietes of the
lateral compartments. In some specimens there are membrane-covered
apertures of considerable size, in others mere narrow clefts, between
the basal halves of the compartments. The basal cup is moderately
deep. The largest specimen was only .16 of an inch in basal diameter.
_Scuta_, rather broad, externally convex, not longitudinally
striated: articular ridge prominent, short, not extending down above
one third of the length of the valve. Depression for the adductor
muscle deep. On the internal surface, near to the rostral angle, a
rather large purple spot of corium adhered to the valve. _Terga_,
broad, externally rather convex: scutal margin protuberant: carinal
margin slightly inflected, or furnished internally with a rim:
articular ridge prominent, very short, not extending down above one
fourth of the valve. Spur very broad, rounded, confluent with the
basi-scutal angle of the valve.
_Internal Structure of the Parietes, Radii, and Basis._--The
parietes, internally, are either quite smooth, or more commonly
ribbed, with the basal edge in consequence crenated; the ribs
are either placed at an unusual distance from each other, and
consequently are few in number, or are pretty close together. The
edge of the basal cup is either quite smooth, or closely crenated,
or distantly toothed, in conformity with the state of the internal
surface of the parietes. The radii have nearly smooth edges, with
their summits more or less oblique. They sometimes extend down only
three fourths, or only two thirds, of the length of the shell, and
the margins of the parietes under the radii being a little hollowed
out, the sutures are converted into clefts or apertures (of course
covered by membrane) like, but not so large as, those in _A.
fenestrata_. The margins of the parietes are hollowed out only on the
side of the radius, and not on both sides of the sutures, as is most
usual in _A. fenestrata_. In some specimens the radii extended down
close to the basal cup, and only very minute clefts were left between
the opposed edges of the parietes.
In the _animal's body_ the only noticeable character was, that on the
anterior ramus of the fourth pair of cirri, some of the segments were
furnished with very broad and thick, small, downwardly curved, teeth
or hooks, like those described in certain varieties of _A. sulcata_;
but they are here stronger and thicker. The segments in the three
posterior pairs of cirri are not so much elongated, as in the other
species.