Chlamys itoigawae Masuda, 1962
MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 33 (2): 117-238, pls. 18-27. [p. 172, pl. 18, figs. 25, 26]
«Chlamys itoigawae Masuda, n. sp.
PI. 18, figs. 25, 26. Holotype: – DGS, Reg. No. 3819.
Description: – Shell moderately thick, higher than long, moderately inflated, subequilateral except for auricles, inequivalve, right valve slightly less convex than left; valves radiately ribbed, apical angle about 95°. Right valve with 26 to 29, rather low, flatly round-topped, smooth radial ribs, smooth but somewhat finely scaled, fine intercalary threads and concentric growth lines; radial ribs somewhat broader than their interspaces, divide into two nearly equal riblets by a shallow longitudinal furrow at about middle part of disc, but radials near submargins usually divide at upper part of disc; interspaces between radial ribs smooth-bottomed, with fine intercalary threads which usually appear on upper part of disc and subequal to divided riblets in strength near ventral margin; secondary, very faint, fine intercalary threads between radial ribs and primary intercalary threads appear on about midde part of disc; concentric growth lines rather distinct near ventral margin; auricles rather large in size; anterior auricle somewhat larger than posterior, with prominent byssal notch and rather wide byssal area, with several, imbricated, fine radial threads and distinct concentric lines; posterior one truncated behind at about right angle, with imbricated radial threads, intercalary threads and concentric lines; hinge with distinct but simple cardinal crura, rather wide and shallow resilial pit provided with distinct lateral ridges which widely open lowards, and conspicuous ctenolium. Left valve moderately inflated, with sculpture nearly similar to right except for radial ribs usually divide at lower part of disc, and fine secondary intercalary threads appear near ventral margin. Interior surface gently folded and with rather distinct, fine marginal serration. Dimensions (in mm.) :- Valve Right* Right Right Left Left Left
Height 20 15 7 22.2 20 14.5 Length 18.8 15 6.5 20.5 18.3 13.2 Hinge-length 10.5 8 4.3 ca.12 11 8 Depth 3.5 2.5 1.2 4 4 3 * — Holotype Comparison and Affinity: – Chlamys nisataiensis Otuka resembles the present new species, but has rather large shell, radial ribs which bifurcate at lower part of disc, intercalary threads which appear on lower part of disc, smaller auricles in which posterior one truncated behind at obtuse angle, rather narrow byssal area in the right valve, and by the left valve having the radial ribs which are narrower than their interspaces and bifurcate at lower part of disc, and a single intercalary thread which appears at about same to as or a little earlier than rib-differentiation. This species differs from Chlamys akitana (Yokoyama) by its larger number of radial rib, faint, fine, secondary intercalary threads, wide byssal area in the
right valve, and by the less inflated left valve and secondary intercalary threads. This species resembles Chlamys ishidae Masuda, but has fewer and more elevated radial ribs which usually divide into two parts at lower part of disc and rather large auricles compared with the present one. Remarks: – This species is named after Dr. Junji Itoigawa of the Institute of Earth Science, Faculty of Science, Nagoya University, who offered the specimens. Type locality, Geological formation and Age: – Floor of the Toki River, about 300 m. upstream of the Relay Station of Wireless Telegraph at Kamado, Mizunami City, Gifu Prefecture (Lat. 35°24'42"N., Long. 137°18'58"E.). Shukunohora formation. Early Miocene. Distribution: – Known only from the lower part of the Shukunohora formation in the environs of Mizunami City. Occurrence: – Common in the very coarse-grained sandstone.» KÔICHIRÔ MASUDA, 1962
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K. Masuda, 1962, plate 18.
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