Mizuhopecten imamurai (Masuda, 1959)
MASUDA, K. 1959. Patinopecten imamurai Masuda, n. sp. from Shimane Prefecture. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 33: 7-8, pI. 1. [p. 7, pl. 1, figs. 7-10]
1959 Patinopecten imamurai Masuda, 1959
K. Masuda, 1959, plate 1.
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«Shell moderate, rather thin, compressed, orbicular in outline, equilateral except for auricles; right valve more convex than left: both valves radiately ribbed and forming an angle of about 100° at apex.
Right valve with eight, low. round-topped, smooth radial ribs and fine concentric growth Iines, and ornamented by obtuse network; radial ribs much broader than their interspaces in central part of disc, and divided into two parts, one being somewhat larger than other; radial ribs at lateral extremities very low and slender, and narrower than their interspaces; bifurcated radial ribs ornamented by very weak, fine longitudinal striae only recognizable by reflected light near ventral margin; interspaces between radial ribs shallow and smooth; anterior auricle sculptured with several radial threads which tend to become obsolete towards margins and concentric lines, ornamented by fine network, and furnished with wide and shallow but distinct byssal notch and rather wide byssal area; posterior auricle similar to anterior in sculpture; hinge with rather distinct ctenolium. Left valve with seven, round-topped radial ribs ornamented by fine longitudinal striae only recognizable by reflected light, intercalary threads and fine concentric growth lines, and ornamented by rather distinct network; radial ribs much narrower than their interspaces, rather sharp near beak, tend to become rounded towards ventral margin, divided into two or three radial threads by shallow longitudinal furrows near beak, but divided radial threads tend to become obsolete downwards; slender intercalary t breads appear at beak and tend to become rounded towards ventral margin; auricles sculptured with several radial threads which tend to become obsolete towards margins and concentric lines, and ornamented by rather distinct network. Characters of hinge area and interior surface unknown. Dimensions (in mm.):— Height 62.5, length 61.5, hinge-length ca. 26, thickness 16.5. apical angle 100°. Comparison and Affinity:— This new species resembles Patinopecten tokyoensis (TOKUNAGA) (1906) from the Pleistocene deposits in the environs of Tokyo, but it can be distinguished therefrom by the smaller shell, the radial ribs which are divided into two parts by shallow longitudinal furrow, the smaller auricles, the ctenolium in the right valve and by the left valve having an intercalary thread between the radial ribs. Also it is distinguishable from Patinopecten kobiyamai KAMADA (1954) from the Jôban Coal-field and Patinopecten chichibuensis KANNO (1957) from the Chichibu basin, by the characters of the radial ribs of the right valve and an intercalary thread of the left valve. Remarks:— This species is named in honor of Prof. Sotoji IMAMURA of the Hiroshima University. The present new species is characterized by its more or less inflated right valve which is provided with eight, round-topped, smooth radial ribs which are broader than their interspaces in breadth and bifurcate at the beak, weak and fine radial threads on the backs of radial ribs, fine net work on the surface, and rather distinct ctenolium. The left valve is characterized by having slightly inflated shell which is provided with narrower radial rihs which are divided into two or three parts by shallow longitudinal furrows ncar the beak but tend to become obsolete towards the ventral margin, an intercalary thread between the radial ribs, and rather distinct network on whole surface. Depository:— Reg. No. TN, N, 1, Geological and Mineralogical Institute, Faculty of Science, Hiroshima University. Locality, Geological formation and Age:— Railroad cutting of the San-in Line at Akazaki, Nima-machi, Nima-gun, Shimane Prefecture. Lat. 3°'00'43''N., long. 132°24'04"E. Kawai formation. Early Miocene. Associated fauna:— Patinopecten kagamianus nimaensis MASUDA, Cyclina lunnulata MAKIYAMA, Crenella fornicata YOKOYAMA, Lucinoma acutilineatum (CONRAD), Thracia hataii KAMADA, Mya cuneiformis (BÖHN), Calliostoma n. sp. Occurrence:— Known only from the type locality.» KÔICHIRÔ MASAUDA, 1959
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