Volachlamys Iredale, 1939
IREDALE, T. 1939. Mollusca. Part 1. In: Great Barrier Reef Expedition 1928-29, B.M.(N.H.), Scientific Reports, 5 (6): 209-425, pls. 1-7. [p. 356]
«Genus Volachlamys nov.
Type: Pecten cumingii Reeve.
Shell of rather Vola-like appearance, but with both valves convex, and with subequal ears, though with Chlamydoid byssal gape and ctenolium. The surface is ribbed, the ribs .smooth, the interstices striate, and no scales appear on the ribs. The edges of the valves are internally grooved to fit, but the grooves do not persist in the interior. The cardinal crura are very weak, a short slender rib running subparallel to the hinge, and a weak ridge on each side of the short broad ligamental pit; the hinge area is obtusely striate. There is a fairly large byssal gape, and a well-marked ctenolium with strong teeth. The juvenile sculpture is very interesting, as the right valve shows a notable smooth umbonal area, while the smooth part in the left valve is restricted to a very small portion, radial ribs beginning very early. This means that a shell of 4 mm. in height would have the right valve smooth and the left valve radially ribbed, though in the adult the sculpture is practically identical in the two valves. The cross-sculpture between the ribs is not developed until a much later stage in the valves, a short space of "Camptonectes" sculpture intervening in the left valve only.»
TOM IREDALE, 1939
|
Pecten Cumingii Reeve; L. A Reeve, 1852-1853, Monograph of the genus Pecten. In: L. A. Reeve (Ed.), 1843-1878, Conchologia Iconica; or illustrations of the shells of molluscous animals, volume 8, plate 31, figure 140.
|
I. Hayami, 1985, Systematics and evolution of Volachlamys from Japan, plate 1. 1a, b. Volachlamys hirasei (Bavay) var. ambigua; 2a, b. Volachlamys hirasei (Bavay) var. ecostata; 3a, b. Volachlamys yagurai (Makiyama) UMUT CM16777a; 4, 5. Volachlamys yagurai (Makiyama) UMUT CM16778a.
|
«Abstract: Volachlamys Iredale, 1939 [type-species: Pecten cumingii Reeve 1853 (= Pecten singaporinus Sowerby, 1842)] is an Indo-Pacifiic pectinid genus (probably a monophyletic group), characterized by Chlamys-like deep byssal notch, persistent ctenolium, relatively large (commonly rectangularly or acutely truncated) posterior wing, undeveloped scales on disk and wings, and generally simple, rarely bifurcated and unornamented radial costae.
Chlamys hirasei Bavay, 1904 is a solitary extant representative of this genus in west Japan and Yellow Sea. Every deme of this species seems to be dimorphic one phenon (var. ambigua) has strong radial costae, whereas the other phenon (var. ecostata) reveals much weaker ribs or nearly smooth surface. Most other shell characters are not significantly different between the two phena, though the ratio of height/length is somewhat larger in var. ecostata, and the shell is considerably heavier in var. ambigua. The two phena seem to be strictly sympatric at least in Osaka Bay and Ariake Bay in west Japan. Chlamys ambigua Bavay, 1904, Pecten awajiensis Pilsbry, 1905, Pecten pulchellimus Tokunaga, 1906, Pecten atsumiensis Yokoyama, 1926, and Pecten teilhardi Grabau and King, 1928 can be all regarded as specifically synonymous with C. hirasei. A rniddle Pleistecene fossil species, Volachlamys yagurai (Makiyama,1924), from the environs of Kobe and west Kyushu is monomorphic, and morphologically resembles Volachlamys singaporina (Sowerby, 1842) now living in the tropical seas of southeast Asia and north Australia. Though much should be further studied, the following evolutionary history is hypothetically presumable: 1) V. singaporina (or its ancestral species) was once widely distributed in the western Pacific, and its northern limit of distribution reached Japan. 2) V. yagurai is an incipient species which was derived from a peripheral isolate of V. singaporina (or its ancestral species) adapted to some inland sea of Japan or its adjacent region (about 500,000 years B. P. or a Iittle earlier). 3) The number of radial ribs on the disk gradually decreased in the course of phyletic evolution from V. yagurai to V. hirasei 4) Every deme of V. hirasei after 130,000 years B. P. came to bear smoothish mutant individuals (i.e. var. ecostata) at a certain relative frequency. At present the frequency is as high as 50 percent both in Osaka Bay and Ariake Bay.» HAYAMI, I. 1985. Systematics and evolution of Volachlamys from Japan (preliminary notes). Venus [Journal of the Malacological Society of Japan], 44: 3-14 (in Japanese with English abstract). [p. 3]
|