Chlamys hataii Masuda & Akutsu, 1956
MASUDA, K. & J. AKUTSU. 1956. A new fossil Chlamys from the environs of Utsonomiya City, Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 21: 129-132, pl. 20. [p. 130, pl. 20, figs. 1-6]
Chlamys hataii MASUDA and
AKUTSU, n.sp. PI. 20, figs. 1-6.
«Shell moderate in size. rather thick, moderately inflated, orbicular, equilateral except for auricles; valves radiately ribbed; pointed at top, forming an angle of about 90º.
Right valve with about 20 elevated, squarish, round-topped, smooth radial ribs and fine intercalary threads; radial ribs wider than interspaces on the upper half of disc and tend to become subequal in breadth near the ventral margin, usually they bifurcate at about middle part of disc, and become a little imbricated at lower half; intercalary threads usually appear at upper half of disc, and are a little imbricated near margin; anterior auricle larger than posterior, furnished with deep byssal notch and more or less wide byssal area. and imbricately ornamented with several radial threads and concentric lines; posterior with a greater number of radial threads than anterior. Left valve nearly equally convex or very slightly more convex than the right and with sculpture similar to that of the right. though the radial threads are less imbricated than the right. Hinge of right valve with ctenolium, distinct cardinal crura and rather deep resilial pit provided with lateral ridges which have straight borders, acutely pointed apically, and rapidly widening vertically. Internal surface nearly smooth, except for characteristic marginal serration. Dimensions (in mm.):--
Type locality amd geological horizon:— Niiya, Tawara-mura, Kawachi-gun, Tochigi Prefecture (lat. 36º 37' 39" N., long. 133º 54' 06" E.). Nagaoka formation, Miocene (Early).
Depository:— Department of Geology, College of Education, Tohoku University, Sendai, Japan. Reg. No. 1370 (Holotype). Remarks:— This species is named in honor of Dr. Kotora HATAI of the Tohoku University. This species is characterized by having about 20 elevated, squarish, round-topped, smooth radial ribs, which are wider than the interspaces in breadth at the upper half of dis, a little imbricated with bi- or tri- or very rarely quadrifurcated radial ribs at lower half of disc, slightly imbricated, fine intercalary threads, deep byssal notch, distinct cardinal crura, conspicuous lateral ridges of resilial pit, and characteristic marginal serration in the right valve. The left valve is characterized by having the sculpture similar to that of the right valve. Although the specimens collected from the Miocene Oido formation at Oido, Wakuya-machi, Tôda-gun, Miyagi Prefecture, are much smaller than the type specimens, with obtuse network and usually smaller proportion of shell height to hinge-length, their external sculpture are similar to hataii (Figs. 7-9). So, it is open to question whether those specimens can be identified with the type specimens of hataii. Further material are necessary to settle this problem. This species is closely refated to Pecten insolitus YOKOYAMA (YOKOYAMA, 1925, p. 18, pl. 5, fig. 3) by having a small shell, about 20 subequal, broad and squarish radial ribs, and an intercalary thread in their interspaces. YOKOYAMA's species was based upon a single right valve, which is said to have been collected from the Shigarami formation at Shimosoyama, Shigarami-mura, Kami-Minochi-gun, Nagano Prefecture. But YOKOYAMA's species differs from the present one by the smaller shell, undivided radial ribs, and hardly developed byssal notch. Chlamys meisensis (MAKIYAMA) (MAKIYAMA, 1926, p. 156. pl. 13. fig. 4) from the Miocene Bankôdô formatioa of Korea, is another reilated species, but it differs from the present specimens by the more inflated left valve and a greater number of radial ribs (23 to 27). Chlamys akitana (YOKOYAMA) and Chlamys nisataiensis OTUKA (MASUDA, 1954, pp. 111-116, pl. 12, figs. 1-17) also resemble the present species, but they can be distinguished from hataii by the greater number of radial ribs (23 to 25), rare occurrence of intercalary thread in the right valve in akitana, and the rather compressed shell, greater number of (25 to 32) and less elevated radial ribs in nisataiensis. Chlamys jordani (ARNOLD) (ARNOLD, 1906, p. 114, pl. 44, figs. 1, 1a-b) described from the Pliocene and Pleistocene formations of California also resembles this species, but it is distinguishable from the present new species, by the greater number of radial ribs (23 to 25), absence of intercalary threads in the right valve, lack of bifurcation of the radial ribs of the left valve.» * Holotype specimen
KÔICHIRÔ MASUDA & JUN AKUTSU, 1956
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K. Masuda & J. Akutsu, 1956,
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