Shell solid, off-white, with relatively high spire. Protoconch is missing on eroded holotype and other shells seen. Teleoconch: spire high, with about four whorls. Last whorl with sculpture of axial ribs (about 18?) and about 8 or 9 spiral threads, forming nodules when crossing. Aperture semi-circular; outer lip thin, slightly flared outward; no internal lirae. Columella straight, with two folds (including rim of siphonal canal) that are solid but not very high. Strong columellar callus field. Umbilicus closed.
Animal pinkish, eyes present, no radula (Bar¬nard 1960).

This is the largest species of the genus as here interpreted. All known shells have only the last whorl and aperture in reasonable condition, the rest of the shell surface is heavily corroded with loss of at least 0.5 mm layer of shell material, and loss of possibly 1-2 mm in the protoconch area. The second S African shell, even more corroded than the holotype, has the posterior fold divided into three close-set minor folds; the anterior fold appears bifid; and in between these primary folds a third sharp fold is placed. Barnard figured a shell with a grooved suture, but in 1989 this was hardly visible on these two shells. The Angola shells are in the same corroded condition; one of them has very faint spiral lines on the last whorl.
The possible relation with B. azorica (which has a multispiral protoconch indicating a pelagic de-velopment) has been mentioned above: in spite of the distance between the Cape Point to Angola and the Azores to Ireland areas, it is possible that they are conspecific. The height/width data of B. decapensis and B. azorica together give a high linear correlation (r2 = 0.99, p < 0.005), but more material of both species, and less corroded shells of B. decapensis are needed for a better evaluation of a possible synonymy.
Barnard (1960: 439) considered B. decapensis "closely comparable with B. nodosa from the New England coast", in spite of the higher spire or the former.

South-Africa: Off Cape Point, 2890-2960 m, and off Ambrizete, N Angola, 1335-1367 m.