Chlamys cosibensis hanzawae (Masuda, 1959)
MASUDA, K. 1959. On the Miocene Pectinidae from the Environs of Sendai; Part 15, Pecten cosibensis Yokoyama and its related species. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 35: 121-132, pI. 13 [p.125, pl. 13, figures 10-15]
1959 Chlamys cosibensis hanzawae Masuda, 1959
K. Masuda, 1959, plate 13.
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«Shell small. inflated. suborbicular, equilaterai except for auricles; right valve more convex than left; both valves radiately ribbed and forming an angle of about 90º at apex.
Right valve with about 20 close-set, rather distinct round-topped, fine radial ribs, finely scaled, fine intercalary threads and concentric growth lines, with distinct fine network; radial ribs in central part of disc usually gathered into four, round-topped, rather elevated, stout fascicular bundles separated by rather deep valleys, usually divided into two parts by a shallow longitudinal furrow towards ventral margin; bundles of four to six, subequal radial ribs broader than their interspaces and sometimes finely scaled, sometimes with a fine intercalary thread between radial ribs; submargins sculptured with several, subordinate, faint, fine radial threads; valleys usually rather deep, narrower than bundles themselves, with unequal intercalary threads which are one to three or rarely more in number, sometimes valleys very shallows and narrow; primary intercalary threads usually appear on upper half of disc, nearly equal to divided radial threads in strength at ventral margin, secondary and tertiary intercalaries somewhat fainter and finer than other radials appear on lower half of disc; when of inconspicuous bundles intercalary threads are one to three in number, and fainter and finer than radial threads; more or less distinct concentric constrictions sometimes present near ventral margin; anterior auricle much larger and longer than posterior one, with narrow and deep byssal notch and rather narrow byssal area, sculptured with several, distinct. more or less imbricated, fine radial threads and concentric lines; posterior auricle similar to anterior, though radial threads somewhat fainter and finer than those of anterior; hinge with very conspicuous cardinal crura, rather wide and shallow resilial pit with distinct, fine lateral ridges widely open towards lower, and with distinct ctenolium. Left valve with about 20, fine radial ribs, three in central part of disc prominent, intercalary threads and concentric growth lines, with rather distinct fine network; three prominent radial ribs rather sharp near beak, subequel to other radials, usually tend to become elevated, stout and rounded towards ventral margin, and divided into three to four or rarely a little more, finely scaled radial threads by shallow longitudinal furrows; other radials finely scaled, rarely divided into two, finely scaled radials by a shallow longitudinal furrow, and nearly equal to or a little less than divided radial threads of prominent radials in strength at ventral margin; intercalary threads finely scaled, primary ones usually appear on about half of disc, and secondary ones appear near ventral margin; submargins sculptured with several, faint. fine radial threads; anterior auricle with several finely imbricated, fine radial threads and concentric lines, and posterior auricle similar to anterior in sculpture, though radial threads fainter and finer than those of anterior. Interior surface of valves gently folded corresponding to external sculpture; characteristic fine serration at ventral margin. Dimensions:— Shown in Table 1. Comparison and Affinity:— This species is distinguishable from cosibensis (s. s.) by its small, suborbicular shell, the more inflated right valve, somewhat larger apical angle, rather distinct, fine radial ribs and indistinct concentric constrictions. Chlamys (Swiftopecten) swftii (BERNARDl) differs from the present one by its large, posteriorly contorted shell, smaller apical angle; radial ribs and very large triangular anterior auricle.
Remarks:— The subspecific name is dedicated to Dr. Shôshirô HANZAWA Professor of the Tohoku University in recognition of his contribution to the paleontology of Japan. Type locality, Geological formation and Age:—Ukibuta, Higashi-Yuri-mura, Yuri-gun, Akita Prefecture. Lat. 39º18'05"N., long. 140º20'05"E. Conglomeratic coarse-grained sandstone of the Sugota formation. Early Miocene. Occurrence:— Sugota formation in Akita Prefecture, Moniwa, Tsunaki, Ôtsutsumi and Oido formations in Miyagi Prefecture, Nanao formation in Ishikawa Prefecture, Suenomatsuyama formation in Iwate Prefecture, and Obashira, Chichibu City, Saitama Prefecture; Early to Late Miocene in age. Depository:— Holotype, DGS. Reg. No. 3690. Paratypes, DGS. Reg. No. 3691 and SM. Reg. No. 7360.» KÔICHIRÔ MASUDA, 1959
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«Remarks: – The present subspecies can be distinguished from cosibensis (s.s.) by its smaller shell, more inflated right valve, somewhat larger apical angle, rather distinct fine radial threads and inconspicuous concentric constrictions.
Uozumi, Fujie and Matsui (1966) illustrated the present subspecies from the Miocene Ainonai Formation, northeastern Hokkaido, based upon a single right valve, but it is open to question whether their specimen can be identified with hanzawae, because the Ainonai specimen is large and its sculpture is more or less different from that of hanzawae. Therefore, it seems that further materials are necessary to clarify the problem. As far as known the present subspecies is known only from the Japanese Miocene formations developed in Northern Japan. Distribution: – Sugota, Moniwa, Otsutsumi, Nanao, Oido, Ogano and Ainonai (?) formations: Early to Middle Miocene.» MASUDA,
K. 1973. Chlamys cosibensis (Yokohama) of the Northern Pacific. Science
Reports of the Tohoku University [2nd. Series - Geology], Special
Volume, 6: 109-116, pls. 8, 9 [Hatai Memorial Volume]. [p. 113, 114]
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Chlamys cosibensis hanzawae Masuda; S. Uozumi, T. Fujie & M. Matsui, 1966, Neogene Molluscan Fauna in Hokkaido: Part III, plate 14. figure 1
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