Parvamussium fenestratum (Forbes, 1844)
British Isles to Cabo verde, lusitanian seamounts to Mediterranean. Shelf depths (ca. 80m-250m in Forbes).
Original taxon: Pecten fenestratus. Synonyms: concentricus, gallimus, inaequisculptus
 
« Pecten fenestratus: testâ minutâ (aequivalvi), orbiculari, costis (5) longitudinalibus, striis (10-15) tansversis, interstitiis minutissimè longitudinaliter striatis ; auriculis aequalibus, magnis, longitudinaliter striatis. » – E. Forbes: “Report on the Mollusca and radiata of the Aegean Sea…”, Report of the 13th meeting of the British Association…, London 1844, via BHL.

140m deep, on bottom with Corallium rubrum, Alghero, Sassari, NW. Sardinia. 6mm. Original pictures provided by M.T. Spanu (IT) – (CC BY-NC-SA).
Pecten inaequisculptus in N Tiberi: Descrizione di alcuni nuovi testacei…, Napoli 1855, via BHL.
 
Nothing is said by Forbes regarding the high differences in the appearance of both valves. This is Tiberi who decribes the best this feature, and peculiarly the sculpture:
« valvae diversimode sculptae ; superior lamellis transversis concentricis radiisque elevatis in longum digestis insignita ; inferior transversim et creberrime striata etc. » – op. cit.
Above and below:
200m deep, off Buggerru, SW. Sardinia. 4,2-4,2mm.
Original pictures provided by A. Nappo (IT) – (CC BY-NC-SA).
The sculpture is typical of the genus.
Left valve at left, right valve at right.
300m deep, 8 miles off Dubrovnik, S. Croatia. 5,1mm. Original picture provided by R. Stanić (HR) – (CC BY-NC-SA).

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