The principal difference between this genus and the Creusia is in form.
Shell sessile, rather globular appearance on account of the valves being
more closely united, ventricose, convex above, apex perforated, aperture
small and elliptical; operculum bivalve.
_P. cancellata._ The cancellated Pyrgoma.
Thick, conical, Patella-shaped, ribs radiating from the summit to the
base; aperture very small, closed by an operculum of which the two
pieces are long and narrow on each side; pale violet colour.