Whitish, slightly depressed, with interwoven obliquely striated valves; the margin at the base irregularly serrated.
6. Pyrgoma. One species.
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.
7. Anatifera. Five species.
The shells of this genus and the three following genera are affixed to marine bodies by a tough membranous peduncle, varying in length.
Shell flat, with five valves imbricating more or less on the edges, united by a thin membrane.
Anatifera lævis.