Mizuhopecten kimurai kimurai (Yokoyama, 1925)
YOKOYAMA, M. 1925. Molluscan remains from the uppermost part of the Jô-ban coal-field. Journal of the College of Science, Imperial University of Tokyo, 45 (5): 1-34, pls. 1-6. [p. 27, pl. 2, fig. 4; pl. 4, figs. 1-6]
1925 Pecten kimurai Yokoyama, 1925
1940 Pecten (Patinopecten) kimurai tiganouraensis Nakamura, 1940
1960 Patinopecten kimurai nakosoensis Masuda, 1960
1960 Patinopecten kimurai yudaensis Masuda, 1960
1940 Pecten (Patinopecten) kimurai tiganouraensis Nakamura, 1940
1960 Patinopecten kimurai nakosoensis Masuda, 1960
1960 Patinopecten kimurai yudaensis Masuda, 1960
M. Yokoyama, 1925, plates 2, 4.
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«Shell large, moderately thick, orbicular, compressed, inequivalve, nearly equilateral. Surface with radiating ribs; ribs on the right valve usually nine in number, broad, rounded, with a few longitudinal riblets or striae on the back, separated by shallow valleys of about equal breadth which are smooth at bottom and show no sharp demarcation against the ribs; ribs on the left valve also usually nine in number, roof-like, sharp with valleys equally formed, that is to say broadly V-shaped. Ears unequal, the anterior one coarsely and radiately corded, with a shallow rounded byssal notch below, while the posterior one is equally long, but usually smooth. Height slightly greater than length; thickness about one-fourth the height.
The largest specimen measures about 100 millim. in height. This shell is much like Pecten tokyoensis Tok. (Yokoyama, Miura Penin., pl. XIV, fig. 7, 8), but the ribs are more prominent and elevated. Fossil occurrence. ─ Izura; Yunami (Tozenji); Sukegawa (Hatsuzaki); Kashjma (Iida and Komeda); Taira (Gomasawa and also near the railway station); Toyoma; Kamitakaku in Iino; Enamura Quarry; Kusano (between Mizushina and Yamazaki); Tamaye, Kobisa.» MATAJIRO YOKOYAMA, 1925
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«Remarks: The specimens from the Toyo’oka Formation have a moderate-sized shell with 6-7 rounded radial ribs in the right valve, and seven roof-topped radial ribs in the left valve. The shagreen microsculpture is visible on the both valves. The interstitial fine radial threads are absent. Based on these shell characters, they are referred to Mizuhopecten kimurai (Yokoyama, 1925a).
The taxonomy of the Mizuhopecten kimurai and its closely related species are controversial. Masuda (1960) classified “Patinopecten” kimurai into the following six subspecies: P. kimurai kimurai (Yokoyama, 1925a), P. kimurai murayamai (Yokoyama, 1926c), P. kimurai ugoensis (Hatai and Nisiyama, 1939), P kimurai tiganouraensis (Nakamura, 1940), P. kimurai nakosoensis Masuda, 1960, and P. kimurai yudaensis Masuda, 1960. On the other hand, Akiyama (1962) discriminated P. (P.) kimurai kimurai, P. (P.) kimurai yudaensis, and P. (P.) kimurai nakosoensis, and separated P. (P.) tiganouraensis and P. (P.) murayamai as distinct species. He subdivided P. (P.) murayamai into four subspecies, P. (P.) murayamai murayamai, P. (P.) murayamai ugoensis, P. (P.) murayamai moniwaensis Masuda, 1958, and P. (P.) murayamai bisecta Akiyama, 1962. Masuda (1963) transferred the members of the Patinopecten kimurai Group (Masuda, 1960, 1962) to the genus Mizuhopecten Masuda, 1963. Ogasawara (1976) added a new subspecies, Mizuhopecten kimurai kagaensis. However, I think these classifications are too much split, because it has been known that a few “subspecies” occur not only in the same stratigraphic horizon in the same area, but also in the same locality (e.g. Masuda, 1960; Akiyama, 1962; Nakagawa, 1998). Therefore, further taxonomical reviews of the “Patinopecten” kimurai Group are needed to clarify their phylogenetic relationships.» MATSUBARA, T. 2011. Miocene shallow marine molluscs from the Hokutan Group in the Tajima area, Hyôgo Prefecture, southwest Japan. Bulletin of the Mizunami Fossil Museum, 37: 51-113, pls. 1-9. [p. 73]
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Mizuhopecten kimurai (Yokoyama, 1925); T. Matsubara, 2011, Miocene shallow marine molluscs from the Hokutan Group, plate 5, figures 6, 13.
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«Remarks:─ About 20 specimens in which fragmental ones are included were collected.
The right valve is ornamented with 9-11 radial ribs which are nearly flat-topped, strongly elevated in the young and are gently rounded in the adult; the left ones have 9-11 distinct radial ribs which are much narrower than their interspaces in breadth and sculptured by obtuse network in the young shells. This species is widely distributed in the Miocene deposits of Japan, and shows a wide degree of variation. The relation to related species of the present one have been discussed by HATAI and NISIYAMA (1939), MASUDA (1955 and 1956) and FUJIE and UOZUMI (1957). Measurements:─ (in mm) Localities and occurrence:─ Loc 2: A roadside exposure, about 1 km SW of Muranokubo, Rokkasho-mura, Kamikita-gun, Aomori Prefecture (Common, fine-grained sandstone); Loc. 3: Cliff of a roadside at Muranokubo, Rokkasho-mura, Kamikita-gun, Aomori Prefecture (Abundant, fine-grained sandstone).
Horizon:─ Upper member of the Takahoko formation (Miocene). Repository:─ T.K.D. Reg. Nos. 5988, 5989, respectively.» AOKI, S. 1959. Miocene Mollusca from the southern part of the Shimokita Peninsula, Aomori Prefecture, Japan. Science Reports of the Tokyo Kyoiku Daigaku [Section C - Geology, Mineralogy and Geography], 6: 255-280, pls. 1-3 [p. 265, 266]
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Patinopecten kimurai (Yokoyama); S. Aoki, 1959, Miocene Mollusca from the southern part of the Shimokita Peninsula, Aomori Prefecture, Japan, plate 1, figure 18; plate 2 figures 1-4.
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