porose

The basis is solid, instead of being porose, in very young specimens of B. improvisus.

Parietes and base porose: shell white, or faintly tinged with purple.

The basis is distinctly porose, by which this species can be distinguished from B. navicula and cymbiformis.

Basis thin, porose; sometimes with an underlaying cancellated layer.

The basis is porose; but I have never seen an underlying cancellated layer of shell, as is so common in several species.