Williamia gussonii
Habitat: it lives in the infralittoral and circalittoral zones.
Distribution: it is collected all over the Mediterranean. Relatively common.
Notes: unmistakable shell, patelliform in outline, brittle, capuliform, ovoid in outline.
Apex not much elevated and placed next to posterior margin. Protoconch has a sinistrorse development, it is globose and it is clearly demarcated from beginning of teleconch. Surface crossed by very fine concentric striae. Peristome sharp and, internally, there are visible muscle scars set up like a horse-shoe. More or less dark brown in colour with darker radial bands. It is present a periostracum thin, bright. Even juvenile specimens are easily recognizable due to globosity of protoconch which is bent towards right. Some varieties such as rubra and major were described by Pallary (1900).
Acmaea curvissior, recently described by Oberling (1970), is considered by CLEMAM as a synonym of W. gussonii. Average measures of adult specimens are around 4-5 mm in length.