Parvamussium undosum Dijkstra, 1991
DIJKSTRA, H. H. 1991. A contribution to the knowledge of the pectinacean Mollusca (Bivalvia: Propeamussiidae, Entoliidae, Pectinidae) from the Indonesian Archipelago. Zoologische Verhandelingen, 271: 1-57, figs. 1-91. [p. 18, figs. 53-61]
1991 Parvamussium undosum Dijkstra, 1991
H. H. Dijkstra, 1991, figures 53-64.
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«Material.— Holotype, sta. 4.033 Tukang Besi Islands, Banda Sea, NW of Binongko, 5°52.5'S 123° 58.5Έ, Van Veen-grab (2x), depth 250-290 m, coarse calcareous sand, 10.ix.1984 (left valve) (RMNH 56550); Paratypes, sta. 4.033 (right valve, additional description) (RMNH 56551); sta. 4.019 Tukang Besi Islands, Banda Sea, S of Kaledupa reef, 5°57.5'S 123°46.5Έ, Van Veen-grab (2x), depth 285-305 m, shell gravel and foraminifera sand, 9.ix.l984 (1 valve) (RMNH 56552); sta. 4.031 Tukang Besi Islands, Banda Sea, NW of Binongko, 5°54'S 123°58.4Έ, Van Veen-grab, depth 390 m, calcareous sand, some shell gravel, 10.ix.1984 (1 valve) (RMNH 56553); sta. 4.032 Tukang Besi Islands, Banda Sea, NW of Binongko, 5°52.5'S 123°58.5'E, Van Veen-grab, depth ca. 385 m, calcareous sand, 10.ix.1984 (1 valve) (RMNH 56554); sta. 4.034 Tukang Besi Islands, Banda Sea, NW of Binongko, 5°52.5'S 123°58.5'E, Van Veen-grab, depth 280 m, coarse calcareous sand, 10.ix.1984 (1 valve) (RMNH 56555).
Description.— Shell small, subcircular and convex, approximately 8 mm both in height and length. Anterior and posterior auricles unequal, umbonal angle about 100°.
The external surface, to about 1 mm beneath the umbonal top, is smooth and glossy with some microscopic scratches. Further on very fine concentric lamellae start, increasing in prominence towards the ventral margin. From about 2 mm beneath the umbo, radial costae begin as well, also increasing in number towards the ventral margin, where about 15 rounded costae are counted. The fine concentric lamellae are developed on the entire exterior shell disc. Both auricles bear also very fine concentric lamellae. The ventral margin is somewhat crenulated, because of the undulated exterior radiation. The interior surface has ten lirae, which are developed to the ventral margin of a distance of 2 mm. The lirae are regularly developed and somewhat pronounced to the end, without any rudimental interstitial lirae. Only two small lirae are visible on the marginal auricles. The hinge line is straight. The cardinal crura is rather broad and the résiliai pit is triangular. A line runs parallel along the periphery of the anterior auricle and ventral margin. The holotype is milky-white, on the inside glossy and semi-transparent. Dimensions: height 8.2 mm, length 8.4 mm. Additional description.— Exterior surface of the entire shell disc with fine concentric lamellae, which are somewhat wider arranged near the ventral margin than elsewhere. The anterior auricle bears about nine small radial costae, and overrunning fine concentric lamellae. Only fine concentric lamellae are present on the posterior auricle. On the interior surface there are ten lirae which nearly reach to the ventral margin, and are somewhat tubercle-like at the end. Two small lirae are visible near the marginal auricles. The byssal notch is small and no ctenolium is present on the suture. The right valve is dirty white, and opaque. Dimensions: height and length 7.9 mm. Etymology.— The exterior surface of the holotype has an undulated appearance. Differentiation.— The most similar species is Parvamussium carbaseum, which is also found at the type locality. P. carbaseum has more radial costae and internal lirae. On the radial costae of P. carbaseum there are small tubercles, which are absent in P. undosum. The sculpture on the auricles of P. carbaseum is more pronounced.» HENK H. DIJKSTRA, 1991
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