Parvamussium sayanum (Dall, 1886)
DALL, W. H. 1886. Report on the Mollusca, Part I. Brachiopoda and Pelecypoda. Reports on the results of dredging, under the supervision of Alexander Agassiz, in the Gulf of Mexico (1877-78) and in the Caribbean Sea (1879-80), by the U.S. Coast Survey Steamer “Blake”…. Bulletin of the Museum of Comparative Zoology at Harvard University, 12: 171-318, pls. 1-9. [p. 214, pl. 5, figs. 3, 9]
1886 Amusium (Propeamusium) [sic] sayanum Dall, 1886
W. H. Dall, 1886, plate 5.
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«Shell compressed, nearly equivalve, somewhat inequilateral; white; with dissimilarly sculptured valves; right valve with (near the middle) about twelve radiating ridges between which toward the base intercalary ridges rapidly appear, so that at the base, in one specimen, there are thirty-two ridges and beginnings of ridges; over the whole are set closely declining concentrically continuous laniellas, uniform, when unbroken, over ridges and intervals; the ears are nearly equal, similarly sculptured to the rest, and with three to five radiating ridges; the cardinal line straight and the umbo not prominent; the left valve is a little smaller, smooth or not ridged, and covered with similar but less elevated and closer lamellae, which swell up in obsolete radiating lines in sympathy with the ridges of the other valve, but which swellings do not cover any genuine ridges; ears subequal, similarly sculptured; byssal notch not deep; interior of valves with 10 to 16 radiating lirae (beside the crura) of which about 10 run home to the body of the valve; ligamental pit moderate, hinge line simple and without transversely rugose areas; max. alt. 15.5, Ion. 15.0, hinge line 7.0, diam. 3.0 mm.
Soft parts yellowish white except the liver; "ocular" tubercles without pigment, but present. Dredged off Morro Light, Havana, at Stations 16 and 100, in 250-400 fms., living, bottom temperature 55°.6 F. A single valve was obtained at Station 143, off Saba Bank, in 150 fms., bottom temperature 63°.5 F. This pretty species is nearest to A. squamigerum E. A. Smith, having the same type of concentric sculpture on the right valve, but differs in the strong rectilinear character of the radiating ribs, in being proportionally wider and more inequilateral, having a wider and less oblique hinge line, and being apparently somewhat flatter. These differences, however, though strong as regards the figured specimens, might be less evident for a large series. Still, they are so marked for what we have, that I have thought it perhaps better to give the present form a separate name than to assume the existence of intermediate varieties, without any intermediate material.» WILLIAM HEALEY DALL, 1886
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«The two genera can be differentiated because Parvamussium has smaller shells than Propeamussium, without lateral gapes, with a well developed byssal notch with larger, more unequal auricles, and with internal ribs that reach almost to the ventral margin of the disc. Propeamussium species are larger and have lateral gapes, no byssal notch, and shorter internal ribs than Parvamussium species, extending for only half to two-thirds of valve height (WALLER 2006b: fig. 5). According to WALLER (1984), Parvamussium sayanum (DALL, 1886) from the Gulf of Mexico has a pseudo-ctenolium and DIJKSTRA (1995) stated that the Indo-Pacific species P. undisonum DIJKSTRA, 1995 is the other species retaining one active tooth in the adult stage.»
DEL RÍO, C. J., A. BEU & S. A. MARTÍNEZ. 2008. The pectinoidean genera Delectopecten Stewart, 1930 and Parvamussium Sacco, 1897 in the Danian of Northern Patagonia, Argentina. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen, 249: 281-295, figs. 1-4. [p. 284]
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