Mizuhopecten chichibuensis (Kanno, 1957)
KANNO, S. 1957. On Some New Species of Patinopecten from the Chichibu Basin, Saitama Prefecture. Bulletin of the Chichibu Museum of Natural History, 7: 101-108, pls. 1-3. [p. 102, pl. 1, figs. 1-5; pl. 2, fig. 5; pl. 3, fig. 4]
1957 Patinopecten chichibuensis Kanno, 1957
S. Kanno, 1957, plates 1, 2, 3.
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«Shell moderate in size, averaging about 80mm. in height (adult), rather variable in outline, inequivalved, nearly equilateral; margins smoothly rounded, antero-and posterodorsal ones rather straight. Surface of both valves with radial ribs crossed by faint concentric Iines of growth, and with microscopic network. Right valve somewhat ventricese with about 5~7 strong, broadly rounded, rather flat-topped radial ribs, most elevated at its central part, separated by flat-bottomed interspaces which are narrower than the width of ribs; of the radial ribs, the most anterior and the posterior ones are low and narrow, sometimes flattened ventrally, and also one of the ribs is ocasionally bifurcated by a narrow and shallow groove; ribs and its interspaces provided with several obsolete radial striae, which become conspicuous towards the base; hinge-line straight, almost one-half shell length in the young and about two-fifths in adult; auricles subequal, but the anterior slightly Ionger than the posterior, and its anterior margin more or less convex, whereas perpendicular in that of the posterior one; surface of both auricles marked with about 5 radial riblets and fine growth lines; byssal notch shallow but distinct; ctenolium weakly developed but distinct on some adult specimens, though indistinct or nearly obsolete in other specimens; cardinal crura distinct, wide and deep resilial pit provided with short, low, fine lateral ridges. Left valve less convex than right, with about 6 rather roof-shaped radial ribs, of which the antero- and postero-dorsal margins are weaker than the others; ribs excepting that of the antero- and postero-dorsal margins provided 2 or 3 radial striae on their backs and in their interspaces at the umbonal area; interspaces between the ribs widely fiat-bottomed, wider than the width of ribs and covered with microscopic network and obsolete concentric growth lines more weakly developed than on the right valve. Apical angle about 100°.
Measurement (in mm.): Comparison and affinity: This new species is closely related to Patinopecten kobiyamai KAMADA (1954, pp. 173~177, Pl. 23, Figs. 1~3) from the Miocene Kabeya formation in the Jôban coal-field, Fukushima Prefecture, with respect to the subcircular shell outline, Iess numerous radial ribs and possessing radial striae on the surface, but the former differs from the latter by lacking the weakly developed and closely spaced intercalary threads on the ventral half of the left valve, and having bifurcated radial ribs, and less convexity of right valve. The present species also bears some resemblance to Patinopecten kimurai (YOKOYAMA), P. kimurai tiganouraensis (NAKAMURA) and P. ugoensis HATAI and NISIYAMA, but may be distinguished from them in the following way. This species may be distinguished from P. kimurai (YOOYAMA) (1925, p. 27, pl. 4, figs. 1~6) from the Miocene Izura formation of Jôban coal-field, Ibaragi Prefecture, by having a less number of radial ribs: from P. kimurai tiganouraensis (NAKAMURA) (NAKAMURA, 1940, p. 13, pl. 2, fig. 4) frorn the Miocene Shiegama formation of Miyagi Prefecture, by having a smaller shell and smaller apical angle; from P. ugoensis (HATAI and NISIYAMA), (1939, p. 40, text-figs.) from the Miocene Yazawagi forrnation of Akita Prefecture, by having a less number of radial ribs, by the Iongitudinal threads on the ribs and the asymmetrical convexity of the valves.
Remarks: The present species more or less varies in its shell outline, namely, some are higher than long, others are longer than high, and rarely a few are obliquely elongated. Moreover, the ctenolium is distinct on some adult specimens, and microscopic network is provided on the surface of right valve in adult shell. Y. KAMADA (1954, p. 176) noticed that the left valve of P. kobiyamai was marked with radial threads consisting of an imbrication of minute scales, and he inclined to refer the features to the genus Vertipecten Grant and Gale (1931, p. 188) and considered it to be the ancestral form of Patinopecten. But, so far as the convexity of both valves is concerned, he pointed out that P. kobiyamai is rather better referable to the genus Patinopecten. It is important to note that the ctenolium is avotive in the genotype of the genera Patinopecten anct Vertipecten. Accordingly the present specimens, so far as the ctenolium is concerned, should not be included under those genera, sensu stricto. The writer tentatively, concludes that the present specimens, are ancestral to the genus Patinopecten. This species may be identical with Pecten (Patinopecten) tuboii OTUKA (MS) of IJIRI ,OGAWA, TAKAZAWA and WADA (1950, p. 23, listed only). Locatities and geological formation: River side near the fall of Nenokami, Yoshida-machi; a small cliff, about 300 meters east of the mentiened fall, Nenokami. Yoshida-machi; about 70 meters in the Iower stream of Gôhei-bashi, Obashira, Ôta-mura; all in the Chichibu-gun, Saitama Prefecture. All from the Iwadonozawa formation (bluish grey fine sandstone), upper OIigocene. Repository: Holotype, Reg. No. 5554; paratype, Reg. No. 5555. Associated fauna: Crassatellites pauxillus YOKOYAMA, Venericardia tokunagai YOKOYAMA, Dosinia chikuzenensis NAGAO, Cultellus otukai OGASAWARA and TANAI, Turritella sp. and Fulgoraria striata YOKOYAMA.» SABURO KANNO, 1957
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