Parvamussium undisonum Dijkstra, 1995
DIJKSTRA, H. H. 1995. Bathyal Pectinoidea (Bivalvia: Propeamussiidae, Entoliidae, Pectinidae) from New Caledonia and adjacent areas. In Bouchet, P. (Ed.), Résultats des Campagnes MUSORSTOM, Volume 14. Mémoires du Muséum national d'Histoire naturelle, 167: 9-73, figs. 1-154. [p. 37, figs. 55-58]
1995 Parvamussium undisonum Dijkstra, 1995
H. H. Dijkstra, 1995, figures 55-62.
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«TYPE MATERIAL. — Holotype MNHN. Paratypes: 5 MNHN, 1 USNM, 2 HD.
TYPE LOCALITY. — Loyalty Islands, MUSORSTOM 6, stn DW 489, 20°48' S, 167°05' E, 700 m.
MATERIAL EXAMINED. — New Caledonia. BIOCAL: stn KG 06, 20°34' S, 166°52' E, 735 m, 2 rv (paratypes: MNHN, USNM).
Loyalty Islands. MUSORSTOM 6: stn DW 488, 20°49' S, 167°06' E, 800 m, 4 Iv (paratypes: 3 MNHN, 1 HD). — Stn DW 489, 20°48' S, 16T05' E, 700 m, 1 spm (holotype). New Hebrides Arc. GEMINI: stn DW 55, 20°59' S, 170°01' E, 710 m, 1 spm, 1 rv (paratypes: MNHN, HO). DISTRIBUTION. — New Caledonia and New Hebrides Arc, 700-800 m, living 700-710 m.
DESCRIPTION. — Shell rather small, inequivalve, inequilateral, up to ca. 14 mm high, somewhat higher than wide, left valve slightly more convex than right, translucent white; auricles nearly equal in size, relatively small, umbonal angle ca. 90°.
Prodissoconch ca. 210 μm in height. Left valve covered with undulating concentric lamellae that cross irregularly spaced radial costae, enlarging towards ventral margin. Auricles with close-set concentric lamellae, which is somewhat more prominent on the anterior auricle. Hinge line straight. Right valve ornamented with regularly spaced concentric Iirae, very weak near submarginal and marginal areas, which are covered by a layer of delicate undulating concentric lamellae (often worn off in dead specimens). Anterior auricle well developed, sculptured with 5 radial riblets, covered with concentric lamellae, which is strongly developed on anterodorsal margin. Posterior auricle covered with concentric lamellae, which are more prominent on postero-dorsal margin. Marginal apron broken off in all specimens seen. Internal lirae 10, 4 of them rudimentary, plus 1 small posterior auricular lira on the left valve; 9 lirae plus 2 rudimentary, and 1 posterior auricular lira on right valve. Lirae commencing near adductor insertion scar and enlarging towards to submarginal area. Exterior sculpture of left valve slightly visible from the interior. Outer ligament small. Resilifer triangular. Byssal notch small. Fasciolar pseudoctenolium present, with one active tooth. Dimensions of the holotype: H 14.2, L 13.9, D 3.1 mm. REMARKS. — The most closely related species is P. sayanum (Dall, 1886) from the Gulf of Mexico and West Indies, which has almost identical sculpture. It differs, however, in having more prominent, radial and more widely spaced concentric lamellae, which are more delicate on the auricles.
WALLER (1984: 213) discussed the pseudo-ctenolium in P. sayanum, which is an unusual feature in Propeamussiidae. P. undisonum is another propeamussiid with an active tooth, only visible in adult specimens. One immature right valve (9 mm high) from BIOCAL stn KG 06 lacks this tooth. No other propeamussiid species known from the Indo-Pacific region approaches P. undisonum. ETYMOLOGY. — From the undulating sculpture (Latin undisonus, adj. = surrounded by wave).»
HENK H. DIJKSTRA, 1995
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