Propeamussium alcocki (E. A. Smith, 1894)
SMITH, E. A. 1894. Natural history notes from H.M. Indian Marine Survey Steamer Investigator, ser. 2 (10). Report upon some Mollusca dredged in the Bay of Bengal and the Arabian Sea. The Annals and Magazine of Natural History [6th Serie], 14: 157-174, pls. 3-5 [p. 172, pl. 5, figs. 15, 16]
1894 Amussium [sic] alcocki E. A. Smith,1894
E. A. Smith, 1894, plate 5.
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«Testa fragilis, subdiaphana, albida, plus minus nitens; valvae compressae; valva dextra fere plana, dilutissime subrosacea, concentrice striata, marginem versus lamellis tenuissimis ornata, sinistra profuudior, albo-pellucida, fere argentea, similiter striata, radiatimque exilissime lirata et striata, radiis tribus paulo prorainentibus posterioribus exhibens: auriculae minimae, subaequales, lincis incrementi, et liris vel striis paucis radiantibus subobsoletis sculptae; umbones acuti, ad angulum circa 115° convergentes; pagina interna nitida, liris undecim haud ad marginem extensis (illis valvae dextra paulo crassioribus) instructa.
Longit. 40 millim., alt. 41, diam. 8. Hab. Laccadive Sea, in 740 fathoms; also west of Andaman Islands, lat. 13° 47' 30" N., long. 92° 36' E., in 561 fathoms.
The three slightly raised rays down the posterior side of the left valve are indicated on the inner surface by corresponding shallow depressions. The largest specimen from the latter of the above localities exhibits rather more concentric lamellation on the left valve than the examples from the Laccadive Sea. This species is very closely allied to A. Watsoni, Smith, but is not so rounded; the right valve has more feeble concentric sculpture, and the radiating lines of the left valve are not so strong.» EDGAR ALBERT SMITH, 1984
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«Description. Shell fragile, hyaline, up to c. 50 mm high, semicircular, inequivalve, inequilateral, left valve slightly more convex than right, prodissoconch c. 240 μm in height.
Left valve with a few commarginal growth lines, and 1–3 delicate radial lirae near posterior margin. Anterior margin more convex than posterior. Auricles equal, smooth, anterior margin somewhat raised. Eight to 10 internal riblets entering from near resilifer to two-thirds length of adult disc, somewhat finer than on right valve; one auricular ridge on each side. Right valve with wide-set commarginal lirae, commencing at c. 3 mm shell length, with granulate interstitial microsculpture. Auricles with narrow commarginal striations, prominent scales on posterior and anterior dorsal margins. Resilifer triangular. No byssal notch. Dimensions. Illustrated specimen, WA, Exmouth, off Yardie Creek, 400–450 m (HMP0234): rv: H 26.1 mm (ventral skirt broken off), L 26.1 mm; lv: H 33.4, L 31.2 mm; D 4.3 mm.
Habitat. Living in the bathyal zone free on soft sediment, muddy sand or sandy mud.
Distribution. Gulf of Aden, 1469 m, Laccadive Sea: 1337–1410 m, Bay of Bengal, 1026 m (Knudsen, 1967); Arafura Sea, Indonesia, 884–1266 m (Dijkstra & Kastoro, 1997; Chesterfield Islands, Coral Sea, 650 m, dead; Loyalty Islands, 780–800 m (Dijkstra, 1995b); New Caledonia, 708–860 m (Dijkstra, 1995b; Dijkstra, 2001); Wallis and Futuna Islands, 1015–1300 m; Vanuatu, 690–750 m (Dijkstra, 2001); Norfolk Island, 949–952 m (Dijkstra & Marshall, 1997), Solomon Islands, 1001–1012 m; Fiji, 427–787 m; Tonga, 824 m (Dijkstra & Maestrati, 2008). Maximum depth range of live-taken specimens is 424–1469 m. Present specimens from Australia alive at 424 m.
Remarks. Propeamussium alcocki is morphologically close to Propeamussium watsoni (E. A. Smith, 1885), which is known from similar localities (Dijkstra, 1995b), but differs somewhat in transparancy (P. alcocki hyaline, P. watsoni opaque and whitish), in shape (P. alcocki slightly more elongate, P. watsoni circular), in texture of the left valve (P. alcocki smooth, P. watsoni with delicate radial sculpture in early ontogeny), and in internal riblets (P. alcocki delicate and weak, P. watsoni more solid).
Knudsen (1967: 281) considered the two names to be synonymous, with variations in shape (circular to almost oblong), commarginal sculpture of the right valve (varies in position), and radial sculpture of the left valve (varies in development) of examined material from the Indian Ocean. These variations are not observed in material from the Southwest Pacific (Dijkstra, 1995b: 13), and we consider these to be two separate species.» 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. 122, 124]
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Propeamussium alcocki (Smith); H. H. Dijkstra & A. G. Beu, 2018, Living scallops of australia and adjacent waters, figures 1A, 1D.
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