Pectinella aequoris Dijkstra, 1991
DIJKSTRA, H. H.. 1992d. Les Pectinidae de Nouvelle-Calédonie/The Pectinidae of New Caledonia. 26. Juxtamusium maldivense (E.A. Smith, 1903). Rossiniana, 54: 24-25. [p. 23, figs. 78-86]
1991 Pectinella aequoris Dijkstra, 1991
H. H. Dijkstra, 1991, figures 78-86.
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«Material.— Holotype, sta. 4.111 Ν of Sumbawa, Bay of Sanggar, 8°19.3'S 118°15.6'E, Van Veen-grab, depth 175-185 m, volcanic sand and gravel with many species of small shells, few living animals, 21.ix.1984 (left valve) (RMNH 56567); Paratypes, sta. 4.111 (right valve, additional description, figured) (RMNH 56568); sta. 4.153 off SW Salayer, 6°22.4'S 120°26.3Έ, Van Veen-grab, depth 130-155 m, calcareous sand with some shells and polychaetes, 28.ix.1984 (3 valves) (RMNH 56569).
Other material studied — Hawaiian Islands, Oahu, off Pokai Bay, dredged, 240-300 m, sandy bottom, xi.1985 (1 specimen, live, coll. Jazwinski) (Dijkstra coll.); Hawaiian Islands, Oahu, off Pokai Bay, dredged, 210 m, in silt and pebbles, vii.1986 (left valve, coll. Jazwinski) (Dijkstra coll.); Fiji Islands, Viti Levu, off Yanuca Island, dredged, 33 m, at base of ledge in sediment, 20.i.1986 (2 right valves, coll. Jazwinski) (Dijkstra coll.). Description.— Shell small, convex, elongate, nearly smooth, opaque; approximately 4 mm in height. Anterior and posterior auricles very unequal, umbonal angle about 90°.
Near the umbonal top the prodissoconch stage and borderline is clearly visible at a magnification of 60 x. Very fine concentric striae increase in prominence towards the ventral margin, with microscopic, radially diverging scratches on the entire shelldisc. The anterior auricle bears several concentric lirae; the posterior one gradually passes into the shell-disc and has very small concentric striae. The internal surface is glossy, and microscopically granulated. Near the border of each auricle one lirai tooth (auricular crura) is developed. The hinge line is nearly straight, and the résiliai pit elongated-triangular. The cardinal crura is rather broad near the pit on the anterodorsal region; a narrow groove runs from the pit to the anterior margin. There is a prominent tooth near the pit on the posterodorsal region; here the groove is smaller and broader than on the anterodorsal part. Shells creamy-white, with a few white and creamy spots. Dimensions: height 4.1 mm, length 3.5 mm. Additional description of right valve (sta. 4.111).— The external surface of the shell-disc has nearly the same microscopic structural features as the holotype; it is also very convex, elongated and smooth. A liral tooth on each border of the auricles is seen on the internal side of the shell. The anterodorsal region of the auricle bears also a straight narrow groove and the hinge line is rised. On the posterodorsal side there is a small basin; the hinge line is straight. The byssal notch is well developed, and neither a ctenolium, nor a pseudo-ctenolium is present. Etymology.— The external surface of the holotype is smooth and glossy. Distribution.— So far only known from Indonesia, the Fiji Islands, and the Hawaiian Islands. Differentiation.— The most similar species is Pectinella sigsbeei (Dall, 1886) from off Havana, Cuba. The microsculpture of P. sigsbeei is somewhat more prominent and Waller (1984: 212) observed 'a weak fasciolar pseudoctenolium' on P. sigsbeei, which is absent in P. aequoris. Shells from the Hawaii Islands and the Fiji Islands are larger and more colourful on the left valve, but otherwise similar. It is possible, that the type material is semi-adult (MCZ 7817, holotype). Remarks.— Waller (1978:353) described a new family Syncyclonemidae, and determined that Pectinella Verrill, 1897 (type species: Pecten (Pseudamusium) sigsbeei Dall) is only a junior synonym of Syncyclonema Meek, 1864. However, in an additional note in the same publication the Syncydonemidae are considered Entoliidae and Pectinella becomes an extant genus in this family, instead of a synonym of Syncyclonema.» HENK H. DIJKSTRA, 1991
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«The species richness of the Syncyclonematinae declined dramatically after the end-Cretaceous extinction. A few species occur in the Palaeocene (Danian) of Greenland, Denmark, and the Chatham Islands (Stilwell, 1998; Heinberg, 1999; Petersen & Vedelsby, 2000), but members of the subfamily are unknown until the late Pleistocene or Holocene of Vanuatu (formerly the New Hebrides; Waller, 1977). The rarity and highly disjunct distribution of Pectinella in the present-day Caribbean and Pacific (Hawaii and New Caledonia) attest to the improbability of the group being found in the Cenozoic fossil record.»
WALLER, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society, 148: 313-342, figs. 1-12. [p. 327]
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