Mizuhopecten naganoensis (Masuda, 1962)
MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series - Geology], 33(2): 117-238 [p. 219, pl. 25, figs. 1a, 1b]
1962 Patinopecten (Kotorapecten) naganoensis Masuda, 1962
K. Masuda, 1962, plate 25.
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«Holotype : — DGS, Reg. No. 3888.
Description: — Shell large, moderately thick and inflated, orbicular, equilateral except for auricles, right valve more convex than left; valves radiately ribbed, apical angle about 110º. Right valve with about 22, rather low, flatly round-topped, smooth radial ribs and fine concentric growth lines; radial ribs broader than their interspaces, usually divide into two or rarely three riblets by shallow longitudinal furrows towards ventral margin or sometimes remain undivided; radial ribs near submargins low and slender, usually remain undivided; auricles extremely large in size; anterior larger than posterior, with fine concentric lines and faint, fine radial threads ; posterior one similar to anterior in sculpture. Left valve with rather distinct, rounded radial ribs, intercalary threads and concentric growth lines; radial ribs narrower than their interspaces, usually rather sharp near beak, tend to become rounded towards ventral margin, usually divide into a few, subequal, fine riblets by shallow longitudinal furrows at about middle part of disc towards ventral margin; interspaces between radial ribs with a few, rather distinct, fine intercalary threads which usually appear on upper part of disc or rarely appear near beak and tend to divide into a few, fine radial threads; intercalary threads rather more conspicuous near submargins than those of central part of disc; auricles with distinct, fine concentric lines and several, fine radial threads which tend to become obsolete towards margins. Dimensions (in mm.)
Comparison and Affinity: — This species differs from Patinopeoten tryblium (Yokoyama) by its extremely large auricles, by the right valve having radial ribs sometimes remaining undivided and by the left valve with low and round-topped radial ribs. Patinopecten yessoensis (Jay) can be distinguished from the present one by its smaller auricles with rather distinct radial threads, undivided radial ribs in the right valve and by the left valve with undivided radial ribs and without intercalary threads. Patinopeoten ibaragiensis Masuda also can be distinguished by its smaller auricles of which the anterior one has a prominent byssal notch, the right valve with undivided radial ribs and by the left valve with undivided radial ribs.
The present one was first illustrated by Yokoyama (19250) and subsequently by Kuroda (1931) from the Shigarami formation at Sakae, Nakajo-mura, Kami-Minochi-gun, Nagano Prefecture as Pecten yessoensis. Type locality, Geological formation and Age:— Floor of a small tributary of the Dojiri River, Shiroshita, Nakajo-mura, Kanli-Minochi-gun, Nagano Prefecture (Lat. 36º37'N., Long. 138º02'58"E.). Shigarami formation. Late Miocene. Distribution: — Known only from the Shigarami formation. Occurrenoe : — Common in the fine-grained sandstone.» KÔICHIRÔ MASUDA, 1962
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