Parvamussium scitulum (E. A. Smith, 1885)
SMITH, E. A. 1885. Report on the Lamellibranchiata collected by H.M.S. Challenger during the years 1873-76. In C. W. Thomson & J. Murray: Report on the Scientific Results of the Voyage of H.M.S. Challenger during the years 1873-1876. Zoology, 13: 1-341, pls. 1-25. [p. 312, pl. 23, figs. 4-4b]
1885 Amussium [sic] scitulum E. A. Smith, 1885
1931 Amusium (Propeamusium) [sic] scitulum var.? cmadoritinctum Kuroda, 1931
1931 Amusium (Propeamusium) [sic] scitulum var.? cmadoritinctum Kuroda, 1931
E. A. Smith, 1885, plate 23.
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«Testa (juvenis ?) parva, compressa, alba, tenuis, subfequivalvis. Valva dextra lineis incrementi paucis hie illic caeteris majoribus instructa, sinistra liris numerosis radiantibus, aliisque minus conspicuis concentricis concinne cancellata. Auriculae mediocres, paululum inaequales, antica valvae dextrae prope marginem obliquum dorsalem canaliculata, lirisque radiantibus paucis subgranulosis ornata, et antica et postica in valva sinistra cancellatis. Pagina interna liris tenuibus circiter denis a marginem productis instructa.
Of this species only a few valves were obtained, possibly representing but the young form of it. It is a thin shell, semitransparent white, a trifle inequivalve, much compressed, and about as long as high. The right valve is slightly deeper than the left, and sculptured only with the concentric lines of growth, of which a few are more raised than others. The left valve, in addition to the raised lines of growth, has a considerable number of fine radiating lirae, giving the surface a more or less cancellated appearance. The beaks are acute, the converging dorsal lines meeting at an angle of about 90 or 95 degrees. The ears are a little unequal, the anterior of the right valve being somewhat channelled close to the oblique dorsal slope and ornamented with a few radiating raised lines which are somewhat granular or subsquamous through being crossed by other raised lines of growth. The posterior auricle in this valve, on the contrary, is almost destitute of sculpture. Both ears in the left valve have radiating and cross lirae, the former predominating, and being, on the anterior auricle, prettily scaled near the dorsal slope. The internal lirae are ten in number and produced to the outer margin, where sometimes they are slightly thickened.
Length 4½ mm., height 4½, diameter 1⅓. Habitat.— Station 188, south of New Guinea, in 28 fathoms; green mud. This species differs from Amussium torresi in sculpture and the number of internal ridges.» EDGAR ALBERT SMITH, 1885
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«Description. — Shell fragile, up to c. 10 mm high, subcircular, compressed, inequivalve, inequilateral, right valve more convex than left, auricles rather small and unequal, umbonal angle c. 120˚, prodissoconch c. 210 μm in height. Left valve stained, opaque, right valve semi-transparent white. Left valve very weakly inflated, weakly sculptured with delicate, variable radial striations, somewhat stronger near posterior margin than elsewhere. Microscopic close-set commarginal striae developed on disc of some specimens, somewhat more widely spaced near umbonal area. Anterior auricle with prominent commarginal lamellae close to flank of disc, decreasing in prominence or absent near antero-dorsal margin; posterior auricle with fine radial and commarginal striations, almost absent from some specimens. Right valve either smooth or weakly sculptured with a few growth lines. Internal riblets extending to submarginal area, 9 or 10 in most specimens, some specimens with 1 or 2 rudimentary interstitial riblets. Resilifer triangular. Hinge line straight. Byssal notch narrow.
Distribution. — Japan to tropical western Pacific: 40-500 m (Hayami, 2000: 913); Indonesia: 50-385 m (dead) (Dijkstra, 1990a: 3; 1991: 17; Dijkstra & Kastoro, 1997: 262); Papua New Guinea: 60-125 m, Chesterfield Islands (Coral Sea): 300 m, New Caledonia: 250-350 m, Loyalty Islands: 70-200 m, Wallis and Futuna Islands: 200-240 m (dead) (Dijkstra, 1995b: 31; 1998a: 14; 2001: 86; Dijkstra & Maestrati, 2008: 94); Fiji: 80-120 m, Tonga: 327-360 m (dead) and the Austral Islands: 200-620 m (dead) (Dijkstra & Maestrati, 2008: 94; 2010: 339). Present material from the Philippines alive in 82-123 m (minimum depth range). Living free on soft bottom assemblages (sand, muddy sand or mud with coral rubble or gravel). Remarks. — The present material from the Philippines is morphologically similar to the type specimens from eastern Indonesia. Kuroda (1931: 77) described a possible new variation of the present species from Japan, which is more colourful and much larger in size than the type material of Parvamussium scitulum. Smith (1885: 312) suggested that the type specimens from New Guinea are possibly juveniles. The present specimens from the Philippines are indeed larger than the type material and the colour is highly variable.» DIJKSTRA, H. H. 2013. Pectinoidea (Bivalvia: Propeamussiidae and Pectinidae) from the Panglao region, Philippine Islands. Vita Malacologica, 10: 1-108, pls. 1-32, 2 tabs. [p. 21, 22]
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Parvamussium scitulum (E.A. Smith, 1885); H. H. Dijkstra, 2013, Pectinoidea from the Panglao region, Philippine Islands, plate 3, figures 2a-2d.
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