Miyagipecten matsumoriensis Masuda, 1952
MASUDA, K. 1952. On some Miocene Pectinidae from the environs of Sendai. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 8: 249-254, pl. 24 [p. 252, pl. 24, figs. 4-7]
1952 Miyagipecten matsumoriensis Masuda, 1952
K. Masuda, 1952, plate 24.
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«Shell moderate in size, thin, subequivalve, equilateral, much compressed, orbicular. Right valve smooth, provided with only fine concentric growth lines; auricles subequal in size, the anterior with concentric lines and wide and shallow byssal notch; internal surface smooth. Left valve with anterior auricle slightly larger than the posterior, marked with radial threads and fine concentric growth lines; surface with 38-42 weak, flatly raised, closely arranged radial threads, which are much broader than their interspaces; radials tend to become obsolete on lower half of disc, and rarely associated with an intercalarly between them; radial threads on younger part of shell usually narrower than their interspaces, but rapidly broaden and flatten towards middle and tend to become obsolete thereon marginally. Internal surface smooth.
Type locality and horizon (of syntype specimens):— The foot of the dam of the water reservoir at Dogasawa, about 700 meters north of Matsumori, Nanakita-mura, Miyagi-gun, Miyagi Prefecture. (lat. 38°19'8'' N., long. 140°55'44'' E.). Nanakita formation. Depository:— Department of Geology, Junior College of Education, Tohoku University. Geological horizon :— Middle Miocene. Remarks:— The radial threads of the left valve usually occupy the upper half of the shell, but in some specimens only one third to one-fourth of the disc is sculptured; rarely they extend to the margin of the shell and in such cases they are very faint and only recognizable by reflected light. In general, small specimens have the radials extending throughout their length. but with continued growth they tend to become obsolete. Placopecten akihoensis (MATSUMOTO) resembles the present species and has been confounded with it, but MATSUMOTO's species can be distinguished from Miyagipecten by the faint radial threads on the surface of the right valve and by them being slightly imbricated, unequal and extending to the shell margin. Further, the interspaces between the radial threads in Miyagipecten are narrower than in Placopecten akihoensis. Placopecten akihoensis, at least in the environs of Sendai, is limited to the Lower Miocene Moniwa formation in geological range, while Miyagipecten matsumoriensis is found only in the Middle Miocene Tsunaki formation (a correlative of the Nanakita formation of the Tomiya District). Thus from their respective geological ranges, it appears that these two species are good guide fossils for the Miocene stratigraphy, at least in the environs of Sendai.» KÔICHIRÔ MASUDA, 1952
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«Discussion.— A left valve and an internal mold of a right valve have been obtained.
The species from the Ichibu Formation has a moderate-sized, amusioid shell with the left valve external surface sculpture consisting of numerous fine radial ribs and very fine antimarginal striae, and the right valve with a shallow byssal notch and auricular crura with a distal denticle. The internal rib carinae and crenulations on the ventral margin are entirely absent. Based on these characters, the Ichibu specimens are safely identified as Miyagipecten matsumoriensis Masuda, 1952. M. matsumoriensis closely resembles Nipponopecten akihoensis (Saga in Matsumoto, 1930), but is distinguished from N. akihoensis by having a right valve lacking radial sculpture, a left valve sculptured by fine radial ribs tending to become obsolete ventrally, and auricular crura with a distal denticle, as already pointed out by Masuda (1952). In addition, the left valve may have antimarginal microsculpture (Figure 11 [sic: figure 10]) instead of shagreen microscupture. Distribution.— Latest middle–late Miocene: Shitazaki and Shimokurosawa formations in Iwate Prefecture (Masuda, 1977; Matsubara, 1996); Matsuba Formation in Akita Prefecture (Masuda et al., 2006); Ôtsutsumi, Nanakita, Aoso, Yutorinuma, Utsuno, Fukuda, Kanagase and Yamairi formations in Miyagi Prefecture (Masuda, 1952, 1957, 1962, 1986b; Masuda and Takegawa, 1965; Masuda and Noda, 1977; Ogasawara and Sato, 1986; Sato and Itabashi, 1999; Sato et al., 2002; Saijo et al., 2009a, b); Ginzan, Hongô and Utsutôge formations in Yamagata Prefecture (Masuda, 1957; Kotaka and Kato, 1979; Ogasawara et al., 1985; Ogasawara and Sato, 1986); Kubota Formation in Fukushima Prefecture (Iwasaki, 1970a; Nemoto and O’Hara, 2003); Ôgane Formation in Tochigi Prefecture (Kurihara, 2010; Kashiwamura, 2011); Senhata Formation in Chiba Prefecture (O’Hara and Ito, 1980; Tomida, 1989, 1996); Misaki Formation in Kanagawa Prefecture (Okumura and Yamagishi, 1992, as from Hatsuse Formation; Hirata et al., 2012); Ichibu and Tsuma formations in Shimane Prefecture (Okubo, 1981; Kakudate, 1988; Sato, 1991; Sato, 2002; this study).» MATSUBARA, T., I. NORO, Y. MATSUURA & T. IRIZUKI. 2014. Miocene Mollusca from the Ichibu Formation on Nishinoshima Island, Oki Islands, Southwest Japan. Paleontological Research, 18 (1): 6-32, figs. 1-11. [p. 23, 24]
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Miyagipecten matsumoriensis Masuda, 1952; T. Matsubara, I. Noro, Y. Matsuura & T. Irizuki, 2014, Miocene Mollusca from the Ichibu Formation on Nishinoshima Island, figures 7.4, 7.8, 10.
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«Miyagipecten matsumoriensis Masuda resembles the present species [Nipponopecten akihoensis (Matsumoto)], but is distinguishable from the latter by its smooth right valve, stouter auricular crura and an intercalary thread between primary threads on left valve (pI. 26, figs. 1-3).»
SATO, Y. 1991. Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, Northeast Japan and description of their Pectinidae. Report, Geological Survey of Japan, 272: 1-249, pls. 1-34. [p. 82]
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Miyagipecten matsumoriensis Masuda; Y. Sato, 1991, Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, plate 27, figures 1-3.
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