Propeamussium watsoni (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. 309, pl. 22, figs. 8, 8a-8c]
1885 Amussium [sic] watsoni E. A. Smith, 1885
1962 Propeamussium watsoni bayonnaisense Okutani, 1962
1967 Amussium [sic] sewelli Knudsen, 1967
1962 Propeamussium watsoni bayonnaisense Okutani, 1962
1967 Amussium [sic] sewelli Knudsen, 1967
E. A. Smith, 1885, plate 22.
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«Testa tenuissima, subdiaphana, albida, paido inaequivalvis, subcircularis, sericato-nitens, utrinque conspicue hians. Valvae compressae, diverse sculptae. Valva dextra profundior, pallidissime fuscescens, liris confertis concentricis prope marginem angustissime lamellatis ornata, sinistra similiter lirata, sed etiam liris gracilibus radiantibus umbones versus praecipue cancellata. Auriculae parvae, incrementi lineis aliisque radiantibus instructae. Pagina interna nitida, marginem versus leviter iridescens, liris albis radiantibus 10-12 baud ad marginem extensis munita.
This charming species is almost circular, very thin and fragile, considerably gaping on both sides above the middle, a little inequivalve, the one valve which I regard as the right being a trifle more swollen than the other. It is also a trifle thinner and of a very light brownish tint. The left valve is of a milky white colour, and consequently rather less transparent. The exterior of the valves is beautifully glossy and has a silky appearance. Both are very finely, concentrically lirate, the lirae towards the outer margin in the right valve being very narrowly lamellated. In addition to this ornamentation the left valve exhibits numerous fine radiating liras, which are decidedly elevated for a short distance around the umbones, and then gradually, further out, take the form of faint substriations. The ears are of moderate size, about equal on each side, and sculptured with elevated lines of growth and others diverging from the beaks, the former sometimes forming a sort of serrate dorsal edge. The umbones are moderately acute, with an angle of divergence of about 125 degrees. The interior is glossy, somewhat iridescent around the margin, and strengthened with from five to six pairs of white riblets of dilferent lengths, the longest becoming obsolete at some distance from the circumference.
Length 50 mm., height 52, diameter 9½. Habitat.— Station 218, north-east of New Guinea, at a depth of 1070 fathoms. This lovely species, one of the prizes of the Expedition, I feel much pleasure in naming after my friend the Rev. R. Boog Watson.» EDGAR ALBERT SMITH, 1885
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«Remarks: The present material from northwestern and southern Madagascar is almost similar to the type material, although the commarginal lamellae of the left valve are almost lacking (only a few near the ventral margin in some of the largest specimens) and the commarginal lamellae of the right valve are somewhat more widely spaced than the typical specimens. However, both features are variable in examined material from other localities in the southwestern Pacific (MNHN).
There are no basic morphological differences between the Japanese morph P. watsoni bayonnaisense and P. watsoni (Dijkstra 1995: 25). According to Knudsen (1967: 280) P. sewelli from the Zanzibar region differs from P. watsoni in having a different shape of the auricles and a different reflection of the posterior margin of the posterior auricle. In addition he also observed differences in the “arrangement of the tentacles of the mantle edge” (Knudsen 1967: fig. 18b, c). However, these three characters of P. sewelli are also observed in specimens of P. watsoni from other localities of the southwestern Pacific (MNHN) and are interspecific variations of P. watsoni. Therefore, P. sewelli is treated herein as a junior synonym of P. watsoni. Knudsen (1967: 281) also treated Propeamussium alcocki (E.A. Smith, 1894) as a junior synonym of P. watsoni, whereas Dijkstra (1995: 13) considered both as valid species. Propeamussium alcocki could be distinguished from P. watsoni by having a semi-transparent thin shell (P. watsoni is opaque and more solid), by having a more oval shape (P. watsoni is almost circular), by having a smooth left valve and sometimes some commarginal lamellae in late ontogeny (P. watsoni has a radial sculpture in early growth stage and more prominent commarginal lamellae in later growth stage), and by having thin internal radial ribs (P. watsoni has more solid and broader ribs, especially on the right valve).» DIJKSTRA, H. H. & P. MAESTRATI. 2015. Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean. African Invertebrates, 56 (3): 585-628, figs. 1-8. [p. 596, 597]
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Propeamussium watsoni (E.A. Smith, 1885); H. H. Dijkstra & P. Maestrati, 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, figures 1K, 1L.
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