1: apex. 2: periostracum. 3: interior of shell. 4: mantle.
5: digestive gland. 6: sole. 7: mantle cavity. 8: anal lobe. 9: head.
10: cleft. 11: tentacular lobe.
Left: when alarmed, W. gussoni emits a copious milky white secretion.
Williamia gussoni (Costa O. G., 1829)
* Usual shell length c. 6 mm. Maximum L. c. 8 mm, W. 6 mm and H. 3 mm (Ruthensteiner, 2006); L. 8.8 mm (P. Ugarković, 2020 pers. comm.)
* Exterior of shell is shiny, bright red-brown with c. 18 pale radiating rays that are sometimes obscured or faded when dead. Periostracum extends well beyond rim of shell.
* Shell interior as exterior, but paler.
* Thin, smooth shell with convex anterior and posterior slopes.
* Mantle is translucent, red-brown with radiating, opaque white rays which correlate with pale rays of shell. No inwardly oriented, large, white repugnatorial glands. In Siphonariidae; respires with c. 17 lamellae concealed in mantle cavity; no pallial gills or protruding ctenidium plume.
* No pallial tentacles. Cephalic tentacular lobes separated from large flat head by small cleft. Large flap-like anal lobe protrudes from mantle cavity on right. Dorsal surfaces of head and foot are red-brown; ventrally yellowish white
* Common at Low Water to more than 50 m (J. Prkić, 2020, pers. comm.) in Adriatic, Mediterranean and adjacent Atlantic; not N.W. Europe (Ruthensteiner, 2006).
* Apex has distinct semi-spiral protoconch positioned off-centre posteriorly, sometimes overhanging beyond the posterior of the aperture.

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