Pecten naganumanus Yokoyama, 1920
YOKOYAMA, M. 1920. Fossils from the Miura Peninsula and Its Inmediate North. Journal of the College of Science, Imperial University of Tokyo, 36 (6): 1-186, pls. 1-19, 1 map. [p. 160, pl. 13, figs. 4-6]
1920 Pecten naganumana Yokoyama, 1920
M. Yokoyama, 1920, plate 13.
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«Shell medium-sized, not very thick but firm, roundish, broader than high, strongly inequivalve, nearly equilateral, the lower margin describing a semicircle with the anterior and posterior corners obtusely angulate and the antero-dorsal and postero-dorsal margins sloping and samewhat concave. Right valve convex and swollen, with broad and flatly rounded, subequal, radiating ribs separated by intervals of a less breadth; number of ribs normally twelve, but sometimes thirteen or eleven with one to three small subsidiary riblets at the anterior as well as at the posterior ends. Left valve quite flat, or even a little concave, also ornamented with flatly rounded radiating ribs which, however, are much narrower than those of the right valve, so that the interspaces become consequently wider than the ribs; their number is normally eleven, but may lessen to ten or increase to twelve and may also be accompanied by a few subsidiary riblets at both ends of the valve. Fine dense concentric elevated lines cross the ribs as well as the interspaces, most distinct on the flat valve, often obsolete on the ribs of the convex valve, on which we often observe several longitudinal striae. Ears are nearly equal especially on the flat valve, the anterior ear of the right valve being marked by a shallow byssal notch.
The largest specimen in our possession measures 82 millim. in height, 95 millim. in length and 25 millim. in depth. This species is very close to the preceding one in the form of the shell, but is at once recognized by the number of ribs. In Pecten laqueatus the normal number of the ribs of the convex valve is eight, which may rarely increase to nine, with or without some subsidiary riblets at both extremities of the shell. Sometimes one or two ribs may be split into two by a median groove, but this groove never attains such a breadth as to make them appear as two separate ones. The left valve has also only eight or nine ribs. Still it may not be impossible that the intermediate forms occur, but so far as my present investigation goes, both on the numerous fossil as well as on recent shells, I have not been able to discover any. Pecten fumatus REEVE (= Pecten bifidus MENKE, PHILIPPI, Abbild., vol. I, Pecten., pl. II, fig. 6 and Syst. Conch. Cab., vol. 7, Pecten, p. 160, pl. 45, figs. 1, 2) from Australia (near Sydney) is also closely related to our fossil, though the beak of the right valve is a little more swollen and the ribs are somewhat wider in the Australian shell. Very frequent. Some of the specimens show traces of an orange-red colour. Fossil occurrence:— Naganuma Zone (Naganuma).» MATAJIRO YOKOYAMA, 1920
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«Only one fragmental shell of right valve was collected from the medium grained sandstone. Although, the specimen lacks the anterior half of shell, it is characterized by flat-topped about 12 major ribs and a few minor ribs on the posterior side. The present form can be identical with Pecten naganumanus Yokoyama from above mentioned character, but left cf. naganumanus untill well preserved specimens are obtained.»
OGASAWARA, K. 1977. Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, Hokuriku Province, Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 47 (2): 43-56. [p. 96]
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Pecten (Notovola) cf. naganumanus Yokoyama, 1920; K. Ogasawara, 1977, Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, plate 8, figure 8.
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«Remarks:– Pecten albicans (Schröter) can be distinguished from the present one by its rather large shell, smaller number of radial ribs, interspaces between radial ribs rarely with a few, faint, intercalary threads and the left valve in younger stage being less concave inwards.
Yokoyama (1928) has stated that his naganumana is a synonym of Sowerby's sinensis (Sowerby, 1947, p. 48, pt 16, figs. 120, 121, 134). However, as pointed out by Kuroda (1932) naganumanus differs from sinensis (= excavatus). With regard to the present one Yabe and Hatai (1937) discussed the detailed geological significances. At the same time they have recorded the present species from the Pleistocene Hiratoko formation developed in the Noto Peninsula, Ishikawa Prefecture, but one of their specimens is referable to Pecten albicans. Type locality, Geological formation and Age:– Road-side cutting at Naganuma, Toyodalllachi, Totsuka Ward, Yokohama City, Kanagawa Prefecture (Lat. 35°22'03"N., Long. 139°32'05"E.). Naganuma formation. Late Pliocene. Distribution:– Naganuma and Ninomiya formations, Kanagawa Prefecture; Shirahama formation, Shizuoka Prefecture; Otadai and Dmegase formations, Chiba Prefecture; Shimajiri formation and Ryûkyû Limestone, Okinawa Prefecture; Malumbang Limestone and Sumagui formation, Philippine; Byôritsu formation, Formosa: Early to Late Pliocene. Occurrence:– Abundant in the medium-grained sandstone of the Naganuma formation; rare in the conglomeratic coarse-grained sandstone of the Shirahama formation.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 33 (2): 117-238, pls. 18-27. [p. 202, 203]
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Pecten naganumana Yokoyama; K. Masuda, 1962, Tertiary Pectinidae of Japan, pl. 23, fig. 10.
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