Parvamussium caesicium Dijkstra & Moolenbeek, 2008
DIJKSTRA, H. H. & R. G. MOOLENBEEK. 2008. Some Pectinoidea (Bivalvia: Propeamussiidae and Pectinidae) from the Berau Islands (East Kalimantan, Indonesia). Venus [Journal of the Malacological Society of Japan], 67 (1-2): 15-26, figs. 1-10. [p. 16, figs. 1-4]
2008 Parvamussium caesicium Dijkstra & Moolenbeek, 2008
H. H. Dijkstra & R. G. Moolenbeek, 2008, figures 1-10.
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«Material examined: NBE stat. 35. Kakaban Island, W-side, 02˚08´21.0˝N, 118˚30´20.1˝E, 10-25 m, scuba diving, 21.X.2003 (paratype: 1 valve, ZMA Moll. 404056). NBE stat. 49. Kakaban Island, NE-side, entrance of tidal lake Kehe Daing, 02˚08´53.2˝N, 118˚32´31.6˝E, 10-25 m, scuba diving, 28.X.2003 (holotype: one left valve, ZMA Moll. 404023; paratypes: 20 valves, ZMA Moll. 404024; 10 valves (MZB).
Type locality: NBE stat. 49. Indonesia, Kalimantan, Berau Islands, Kakaban Island, NE-side, entrance of tidal lake Kehe Daing, 02˚08´53.2˝N, 118˚32´31.6˝E, 10-25 m, scuba diving, 28.X.2003. Description: Shell small, approximately 3 mm in height and length, fragile, circular, inequivalve, rather inflated, left and right valve equally convex, anterior auricle larger than posterior, umbonal angle about 110˚, colour whitish, transparent in early growth stage, semi-transparent later. Left valve sculptured with widely spaced, delicate commarginal lamellae in early growth stage, commencing at 0.3 mm shell height, extending to 1 mm shell height, increasing by finer lamellae and much smaller spacing to ventral margin. Radial riblets (ca. 15) commence at ca. 1 mm shell height and become stronger to the ventral margin (ca. 30). Radial ribs bear small nodules. Anterior auricle with prominent commarginal lirae (ca. 12), weaker on posterior auricle, smaller in number (ca. 8) and more closely spaced. Right valve with regularly arranged, very closely spaced commarginal lirae. Anterior auricle much larger than posterior with 3-5 radial riblets, posterior auricle with antimarginal microsculpture. Hinge line straight. Byssal notch shallow, byssal fasciole and ctenolium lacking. Ten or eleven internal ribs plus a few rudimentary and one weak auricular on each side, commencing at 1 mm shell height and extending close to ventral margin. Dimensions of the holotype: height 3.2 mm, length 3.2 mm. Etymology: The sculpture of the left valve looks like a fine closely woven white curtain (Lat. caesicius, adj. = closely woven). Discussion: Parvamussium caesicium n. sp. closely resembles in sculpture P. carbaseum Dijkstra, 1991, known from bathyal depths in the Banda and Flores Seas (Indonesia), but differs by lacking secondary intercostal radial riblets (1-4 on the left valve of P. carbaseum), by the more closely spaced commarginal sculpture on the anterior auricle of the left valve, by a more transparent early growth stage (opaque in P. carbaseum) and by the number of internal ribs (10-11 plus 1-2 rudimentary of P. caesicium n. sp, 13 plus several rudimentary in P. carbaseum). Parvamussium undosum Dijkstra, 1991, known from bathyal depths in the Banda Sea (Indonesia), also has very closely spaced commarginal lamellae, but these also cover the radial ribs. Moreover, these ribs are much broader and smaller in number than those of P. caesicium. n. sp.» HENK H. DIJKSTRA & ROBERT GERARD MOOLENBEEK, 2008
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