Shell dirty reddish-purple, steeply conical: radii narrow: basis obscurely porose. Scutum with the lines of growth crenated: tergum with the spur truncated, broad as half the valve, and depending beneath the basi-scutal angle as much as half its own breadth.
Hab.—Japan, attached to an Isis, Mus. Cuming. Attached to an oyster, Mus. Stutchbury.
As already stated, this species comes in all essential respects very near to the last, though differing much in appearance; I have seen two sets of specimens, and two sets of B. allium, and there was no variability or passage in the points in which they differed; hence I must consider them as specifically distinct.
Shell, steeply conical, strongly but bluntly ribbed longitudinally; coloured either all over dull reddish purple, or with the upper part only pinkish purple: in one set of specimens, the yellow epidermis was partially persistent. Radii narrow. Orifice small, ovate. The wall of the carino-lateral compartment is very narrow. The internal surface of the parietes is ribbed, but finely, and only in the lower part. The septa, on the sutural edges of the radii, are finer than in B. allium. Basis flat, obscurely permeated by pores. The largest specimen is .25 of an inch in basal diameter.
Scuta: these are longitudinally and finely striated; the basi-tergal corner is more rounded off than in B. allium, and the articular ridge is not nearly so prominent: internally, the adductor ridge is rather more prominent. The Tergum is rather broader: its apex is produced into a minute sharp point: the scutal margin is straight; the spur is broader, and measured from the basi-scutal angle of the valve, considerably longer; namely, as long as half the width of the basal margin of the spur, whereas in B. allium it is only about a quarter as long as the basal margin of the spur: the lower edge of the spur is not here so directly transverse to the longitudinal axis of the valve as in B. allium: the external surface is not so flat as in that species, and a depression runs down to the basi-scutal angle of the spur.
Considering the difference in the shape and appearance of the shell, with its narrow radii and small orifice; considering the less strongly ribbed internal lamina of the parietes, the finer septa on the sutural edges of the radii, the slight difference in outline in the scuta and terga, more especially the greater length of the spur, I conceive I am right in ranking this form as a distinct species, though assuredly it is very closely allied to B. allium, and even still closer to the following B. quadrivittatus.