Parvamussium thetidis (Hedley, 1902)
HEDLEY, C. 1902. Scientific results of the trawling expedition of H.M.C.S. Thetis off the coast of New South Wales in Febr./March 1898. Part I. Brachiopoda and Pelecypoda. Australian Museum Memoir, 4: 287-324, figs. 39-60. [p. 304, fig. 49]
1902 Amusium thetidis Hedley, 1902
1905 Pecten (Amussium) [sic] cristatum Bavay, 1905
1912 Amussium [sic] cristatellum Dautzenberg & Bavay, 1912
1929 Ctenamusium salacon Iredale, 1929
1905 Pecten (Amussium) [sic] cristatum Bavay, 1905
1912 Amussium [sic] cristatellum Dautzenberg & Bavay, 1912
1929 Ctenamusium salacon Iredale, 1929
Parvamussium thetidis (Hedley, 1902); H. H. Dijkstra, 1995, Bathyal Pectinoidea (Bivalvia: Propeamussiidae, Entoliidae, Pectinidae) from New Caledonia and adjacent areas, figures 99-102.
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«Valve small, thin, white, translucent, compressed, equilateral, externally glossy and concentrically grooved by reverse imbricating sculpture, the internal lyrae visible through the shell. The anterior auricle well developed, without ctenolium. Hinge line long, slightly concave, umbo projecting. Within are ten slender raised radiating lyrae, knobbed at the ventral margin and vanishing towards the umbo. A pair of auricular crura occur on either side of the hinge. The hinge plate and ventral margin are roughened by fine vermiculate scratches. Chondrophore minute, triangular, the depth of the hinge plate. Length, 6; breadth, 6 mm.
This seems to represent the A. lucidum, Jeffreys, of Northern Seas.
One perfect right valve and several fragments were taken off Port Kembla in 63-75 fathoms.» CHARLES HEDLEY, 1902
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«Description. Shell fragile, small, up to c. 10 mm high, opaque, inequivalve, inequilateral, left valve slightly more convex than right, auricles unequal in size, umbonal angle c. 95°, prodissoconch c. 210 μm high. Left valve cream, some specimens with white spots; right valve uniform whitish. Internal riblets 8–10, most specimens with 10, some specimens with rudimentary interstitial riblets, plus 1 or in a few specimens 2 auricular riblets. Riblets fine at first, becoming more prominent distally near ventral marginal.
Left valve sculptured with unevenly spaced radial riblets, commencing at c. 1 mm shell height, enlarging to ventral margin. Commarginal lamellae rather delicate, variable, close-set to more widely spaced, traversing radial riblets. Auricles with fine, closely spaced commarginal lamellae, somewhat weaker on posterior than anterior auricle. Dorsal margin straight. Right valve with regular, rather widely spaced commarginal lirae. Anterior auricle with closely spaced commarginal lamellae and 1–3 weakly developed radial riblets near byssal fasciole, posterior auricle with commarginal lamellae only. Dorsal margin slightly raised at ends, with small scales along margin. Resilifer triangular. Byssal notch narrow. Habitat. Living on the continental shelf and in the bathyal zone, free on soft sediment (sand, muddy sand or sandy mud).
Distribution. Philippines, 80–150 m [as Parvamussium cristatellum] (Dijkstra, 2011); Eastern Indonesia, 230–467 m [as P. cristatellum] (Dijkstra, 1991; Dijkstra & Kastoro, 1997); Chesterfield Islands, Coral Sea, 500–540 m; Norfolk Ridge, 573–650 m; Loyalty Islands, 310–350 m (dead); Vanuatu, 450–550 m (dead) [as P. cristatellum] (Dijkstra, 1995b); Kermadec Islands, 256–549 m (dead) [as P. cristatellum] (Dijkstra & Marshall, 1997); Norfolk Ridge, 569–616 m; Fiji, 390–410 m; Tonga, 351–450 m (dead) (Dijkstra & Maestrati, 2008); Austral Islands, 140–750 m (dead) [as P. cristatellum] (Dijkstra & Maestrati, 2010). Maximum depth range of live-taken specimens is 230–650 m, from Australia 110–1161 m.
Remarks. The external sculpture of the left valve of Parvamussium thetidis is extremely variable. The primary radial riblets vary in prominence from strongly developed to very weakly developed or even lacking on the central part of the disc and are unevenly spaced. The commarginal lamellae also vary in spacing from very closely to more widely set. All these variations are observed in the abundant material from around Australia. This includes material indistinguishable from the specimens previously identified as P. cristatellum (Dautzenberg & Bavay, 1912) and Ctenamusium salacon Iredale, 1929, which we therefore regard as a synonyms of P. thetidis.
The northwestern Indian Ocean species Parvamussium siebenrocki (Sturany, 1901) and Parvamussium formosum (Melvill in Melvill & Standen, 1907) are very closely similar to the present species (see Dijkstra & Marshall, 1997: 80) and could be morphs or even synonyms of the one species, in which case the earliest name for this species would be P. siebenrocki. However, the distribution and range of variation require further study before this is accepted.» DIJKSTRA, H. H. & A. G. BEU. 2018. Living scallops of Australia and adjacent waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae). Records of the Australian Museum, 70 (2): 113-330, figs. 1-102. [p. 144]
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Parvamussium thetidis (Hedley); H. H. Dijkstra & A. G. Beu, 2018, Living scallops of australia and adjacent waters, figures 12A-12I.
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