Formerly classed with the Bulla, but separated from it on account of the
shell being entirely covered by the animal, and never externally
visible.
Shell oval, thin, fragile, more or less involuted on one side, rendering
the aperture more or less wide.
_B. aperta._ The open Bullæa. Pl. 17, fig. 5.
Shell interior and very incompletely involuted, without spire or
columella; sub-orbicular, white, transparent, faintly striated, and
slightly wrinkled; almost entirely open.