Tetrarca tetragona (Poli, 1795)
Norway to Canarias, Azores & Madeira to Mediterranean. Also registered in Angola (Gofas & al., 1985). Intertidal (UK) or infralittoral (circa 35m) down to 200m deep, attached on hard substract or large shells by a strong byssus.
40m deep, Málaga, Andalucia, S. Spain. 28mm.
Synonyms: brittanica, cardissa, fusca.
30-50m deep, off Almería, Andalucia. 30-31mm.
« Shell oblong, very inequilateral, with costated striae, decussated by distinct lines of growth ; anterior side rounded, posterior angulated, with a prominent keel or ridge from the umbo to the posterior ventral margin ; umbones distant, incurved ; ventral margin gaping. » – S. Wood: A monograph of the Crag mollusca vol. II, London 1851-1861 via BHL.
« Arca Britannica – Shell obliquely trapeziform, sides angulated at the upper part, anterior very short, attenuated, posterior obliquely angulated, acuminated at the lower part, with a prominent keel running from the umbone to the acumination ; light brown ; decussated with transverse and longitudinal striae ; area of the ligament rather broad, umbones curved inwards. » – L. A. Reeve: Conchologia iconica vol. II, London 1843, via BHL.
Arca tetragona in J. G. Jeffreys: British Conchology vol. V, London 1869, via BHL.
An old specimen from Málaga. 26mm.
The animal in Forbes & Hanley: A history of British Mollusca and their shells vol. I, London 1853, plate P.
Hairy young specimen from 35m deep, Saronic Gulf, Attlikí, E. Greece. 14,35mm.

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