Azumapecten iwakiana (Yokoyama, 1925)
YOKOYAMA, M. 1925. Molluscan remains from the middle part of the Jô-ban coal-field. Journal of the College of Science, Imperial University of Tokyo, 45 (7): 1-23 [p. 19, pl. 3, fig. 12]
1925 Pecten iwakianus Yokoyama, 1925
M. Yokoyama, 1925, plate 3.
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«A fragment of a large valve which, however, shows such a peculiar sculpture as not to be confounded with any hitherto described.
The shell is nearly 80 millim. long, strongly compressed and furnished with more than twenty elevated, rounded, radiating ribs, narrower than the valleys in each of which there is a fine riblet. The ribs as well as the riblets are closely scaled. The species comes closest to Pecten crassicostatus Sow. (Yokoyama, Foss. Miura Penin., pI. XII, fig. 7) living and fossil in Japan. This latter, however, has the ribs broader than the valleys and more coarsely and distantly scaled. Fossil occurrence.— Tateishi Region: Between Tateishi and Shiogu.» MATAJIRO YOKOYAMA, 1925
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«One incomplete left valve and several fragmentary specimens exhibit some diagnostic features of this species which was first described by Yokoyama (1925) from the Hirono Formation in the Tateishi area near this fossil locality. The left valve is slightly inflated and has 22 strong roundly flat-topped scaly radial ribs, alternating with riblets which become less conspicuous in the anterior half of the disc. Interspaces are nearly equal in breadth to the ribs and are ornamented with 3-4 fine threads near the ventral margin of the disc. The anterior auricle is triangular and ornamented with riblets and threads. The hinge has a wide resilial pit and simple cardinal crurae. The interior surface is weakly folded in correspondence with the radial ribs.
This species is distinguished from Chlamys farreri nipponensis Kuroda, which is widely distributed in modern shallow waters and in younger Cenozoic formations of Japan, by its smaller number of ribs and imperfect differentiation of the second- and third-order radials. It occurs commonly in the Taga Group on the Pacific coast of Ibaraki Prefecture (Omori and Suzuki, 1950; Masuda, 1962). It has also been recorded from the Daishaka Formation of Aomori Prefecture and from the Setana Group of Oshima Peninsula in Hokkaido (Nomura and Hatai, 1935; Kanehara, L942).» O'HARA, S. & N. NEMOTO. 1988. Pectinids from the Taga Group of the Joban Coalfield. Saito Ho-on Kai Special Publication, Professor Tamio Kotaka Commemorative Volume: 481-496, pls. 1-4. [p. 488]
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Chlamys iwakiana (Yokoyama); S. O'Hara & N. Nemoto, 1988, Pectinids from the Taga Group of the Joban Coalfield, plate 2, figure 6.
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Comparison: Chl. (Leochlamys) daishakaensis is considered the nearest species, which, the Japanese paleontologists confused for a long time with the described species, which according to the view of K. Masuda, was related to the fragmental representation of Ch. (Leochlamys) iwakiana Yokoyama (1925) Chl. (Leochlamys) iwakiana is differentiated from ChI. (Leochlamys) daishakaensis by the character of the radial ribbing; which in the latter consists of separated and sometimes conjugated thin and high ribs with a rounded cross section at the top and with a crossed base, while in ChI. (Leochlamys) iwakiana, ribs at the base are slightly enlarged and consist of thin radial 3 or 4 riblets, especially well-marked in the younger specimens. Imbrication in the latter species is much smaller than in Chl. (Leochlamys) daishakaensis.
Age: Miocene-early Pliocene Location: Japan, prefektura of Fukusim, railway depression between Tateish and Shiogai; Kamchatka, section between promontory Bolshim and the mouth of Pustoi river (collections of A.S. Arsanov Yu. G. Drushits). Distribution: Formations of Futab-Tomiok, Fukusom and Khitasy of Japan; Etolon suite west of Kamchatka. SINELNIKOVA,
V. N. 1975. Mio-Pliocene Pectinidae of Kamchatka [Pektinidy mio-pliotsena Kamchatki]. Transactions of the Geological Institute,
Academy of Sciences of the USSR, 229: 1-140, 25 pls.
(in Russian with English title and contents; Translated for the U. S.
Department of the Interior, Geological Survey, and the National Science
Foundation, Washington, D. C., by the Al-Ahram Center for Scientific
Translations, 1977). [p. 73, 74]
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Chlamys (Leochlamys) iwakiana (Yokoyama); V. N. Sinelnikova, 1975, Mio-Pliocene Pectinidae of Kamchatka, plate 15, figures 1a-1d, 2.
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