Amussiopecten akiyamae Masuda 1962
MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series - Geology], 33(2): 117-238. [p. 224, pl. 27, figs. 1-3]
1962 Amussiopecten akiyamae Masuda, 1962
K. Masuda, 1962, plate 27.
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«Holotype : — DGS, Reg. No. 3854.
Description: — Shell large, moderate in thickness, compressed, somewhat longer than high, equilateral except for auricles; right valve more convex than left; valves radiately ribbed, apical angle about 110°. Right valve slightly inflated, with about 17, very low, broad, rounded radial ribs and rather distinct, fine concentric growth lines; radial ribs broader than their interspaces, gradually tend to become obsolete towards ventral and lateral margins, with very faint, fine longitudinal striae near ventral and lateral margins; radial ribs near submargins much lower and more slender than those of central part of disc; interspaces between radial ribs smooth, but with very faint, fine longitudinal striae recognizable by reflected light on lower part of disc; auricles rather large in size, subequal to each other, with distinct, fine concentric lines; anterior with very wide and shallow byssal notch. Left valve nearly flat or slightly inflated, but slightly concaved inwards in younger stage, with low, round-topped radial ribs and rather distinct, somewhat lamellated, fine concentric growth lines; radial ribs narrower than their interspaces in breadth, rather sharp on upper part of disc but tend to become rounded and obsolete towards ventral margin and nearly equal to their interspaces near ventral margin; radial ribs near submargins very faint and low; submargins sculptured with very faint, fine radial threads. Interior surface with prominent internal ribs. Dimensions (in mm.): —
Comparison and Affinity: — The present species can be distinguished from Amussiopecten praesignis (Yokoyama) by its low, rounded radial ribs which never divide in the right valve and by the left valve having rather narrow, low radial ribs. Amussiopecten iitomiensis (Otuka) can be distinguished from the present one by its fewer and more prominent radial ribs which are not sculptured with fine longitudinal striae, and the interspaces between radial ribs with no intercalary thread.
Remarks: — This new species has been described by Akiyama (1957) from the Zushi formation as Amussiopecten planicostulatus (Nomura and Niino), but it is evident that the present one is not a synonym of planicostulatus which belongs to the genus Patinopecten. Type locality, Geological formation and Age: — Sea cliff at Fudoiwa, Kanaya, Amaha-machi, Kimitsu-gun, Chiba Prefecture (Lat. 35°09'30"N., Long. 139°49'30"E.). Inagozawa formation. Middle Miocene. Distribution: — Inagozawa formation, Chiba Prefecture; Zushi formation, Kanagawa Prefecture: both Middle Miocene in age. Occurrence: — Few in the granule conglomerate of the Inagozawa formation.» KÔICHIRÔ MASUDA, 1962
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