Parvamussium cassium 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. 11, figs. 22-27]
1991 Parvamussium cassium Dijkstra, 1991
H. H. Dijkstra, 1991, figures 22-32.
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«Material.— Holotype, sta. 4.142 NE Taka Bone Rate (Tiger Isl.), Ε of Tarupa Kecil, 6°29.7'S 121°10.8Έ, rectangular dredge, depth 450-600 m, calcareous mud, partly hardened, Epizoa, 26.ix.1984 (1 live specimen) (RMNH 56549).
Description.— Shell small, subcircular, left valve slightly more convex than right one, approximately 10 mm in height. Anterior and posterior auricles very unequal (anterior 4 mm, posterior 2.5 mm); umbonal angle about 120°. Outline as in Delectopecten. Beneath the umbonal region of the external surface of the left valve, there is a cancellated sculpture with equally strongly developed fine radial and concentric costae, and microscopic scratches on the interstices. Many fine spines are set on the intersections of the cancellated sculpture, most strongly developed near the periphery. Both auricles bear also a cancellated sculpture. The sculpture on the anterior auricle is slightly more pronounced.
The exterior surface of the right valve has concentric lamellae, which are somewhat crenulated near the ventral margin, because of microscopic radial scratches on the lamellae. The anterior auricle has about eight radial costae, with several fine concentric lamellae, which are strongly developed on the dorsal margin, which has a serrated appearance. Only a few small radial costae are seen on the posterior auricle; more dominant are the concentric lamellae. The exterior cancellated sculpture is visible on the interior glossy surface of the left valve. The interior surface is somewhat undulated near the ventral margin, caused by disturbed development of growth. There are only two lirae on the border of the auricles. Both the anterior and the posterior margins of the auricles are somewhat spiniferous. The hinge line is straight. The resilial pit is oblong and ochreous coloured. The interior surface of the right valve is also transparent and glossy. Only one small lira is seen on the posterior auricle. On the anterior dorsal hinge line many scales are visible; these are less clearly developed on the posterior region. The byssal notch is small and the byssal gape almost absent. There is no ctenolium on the suture. The shell is white and semi-transparent. Dimensions: (left valve) height 9.9 mm, length 10.5 mm; (right valve) height 8.9 mm, length 9.1 mm; diameter 2.1 mm. Etymology.— The exterior surface of the left valve has an appearance of a fine net. Differentiation.— A closely related species is Parvamussium vidalense (Barnard, 1964) from South Africa, which differs by more coarsely developed concentric lamellae and squamae on the radial costae of the left valve. The auricles are also more prominently sculptured. There are some lirae on the interior surface, which are absent in P. cassium, except for the two lirae on the border of the auricles. Another similar species is Parvamussium araneum, which is more coarsely sculptured with fewer radial and concentric costae on the left valve. The auricles are also more strongly sculptured, and the concentric lamellae are not crenulated near the ventral margin. Some rudimental lirae are seen on the interior surface; these are absent in P. cassium. Both similar species are somewhat smaller.» HENK H. DIJKSTRA, 2001
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