Azumapecten niikappuensis (Masuda, 1962)
MASUDA, K. 1962. Tertiary Pectinidae of Japan. The Science Reports of the Tōhoku University [2nd. Series - Geology], 33 (2): 117-238, pls. 18-27. [p. 180, pl. 21, fig. 5]
1962 Chlamys niikappuensis Masuda, 1962
K. Masuda, 1962, plate 21.
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«Holotype: -- DGS, Reg. No. 3835. A single left valve.
Description: — Shell thick, higher than long, moderately inflated, nearly equilateral except for auricles; valveradiately ribbed, apical angle about 80°. Valve rather irregularly inflated, with about 13, strongly elevated, squarish, distinctly imbricated radial ribs, imbricated intercalary threads, concentric growth lines and obtuse network; radial ribs narrower than their interspaces; interspaces between radial ribs sometimes with imbricated, elevated intercalary threads which usually appear on upper part of disc; scales heavy, arched, inclined apically; auricles large in size; anterior auricle larger than posterior, furnished with more or less distinct byssal sinus, sculptured with imbricated radial threads and concentric lines; posterior auricle and hinge area unknown.
Dimensions (in mm.): — Height 65, length ca. 48, depth ca. 15.
Comparison and Affinity: — The present species resembles Chlamys foeda (Yokoyama) in having about 13, squarish, elevated, strongly imbricated radial ribs, but differs by having imbricated intercalary threads and no lamellated sculpture in the interspaces.
Remarks: — Since the description of this species is based upon a single, somewhat imperfect left valve, the characters of the right valve and the detailed features of the left valve remain unknown. However, no species similar to the present one has been described from the Cenozoic deposits of Japan. This species may be ancestral to foeda.
Type locality, Geological formation and Age: — Cliff of the left side valley of the Kami-Futamata-zawa, upper course of the Biu River, about 5.5 km. northeast of Biu, Niikappumachi, Niikappu-gun, Hidaka Province, Hokkaido (Lat. 42°32'52"N., Long. 142°26'54"E.). Noya formation. Early Miocene.
Distribution : — Known only from the type locality.
Occurrence: — Rare in the conglomeratic very coarse-grained sandstone.»
KÔICHIRÔ MASUDA, 1962
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«Remarks:
Ozaki (1958) described Chlamys (Mirapecten) robustus as a new species from the Na-arai Formation in Chiba Prefecture. According to his description, this "new species" differs Chlamys foeda by having a flatter shell, narrower apical angle and more numerous ribs (15 in robustus and 13 in foeda). However, some specimens of the foeda from the Shigarami. and the Hitachi Formations have 15 radial ribs and show nearly same apical angle to the robustus. Therefore, it is hard to distinguish the robustus from the foeda. Affinities:
The present species is closely related to Chlamys (Chlamys) niikappuensis (PI. 1, fig. 5) from the early Middle Miocene Noya Formation in Hokkaido described by Masuda (1962). The latter species has 12 strong and imbricated radial ribs. However, the interspaces of the latter are ornamented with 4 radial ribs, 5 weak radial threads and network sculpture which are never observed in the foeda. Chlamys chiyodai, described from the Middle Miocene Otsutsumi Formation in Miyagi Prefecture, is another allied species to the present one. According to Masuda and Sato (1977) who proposed the chiyodai as a new species, this species can be distinguished from the foeda by its few, fine riblets on the radial ribs in the right valve and less lamellated growth lines. Surely, the above two Miocene forms are discriminated from the present one. However, both description of these forms were based on only one specimen respectively. Therefore, it is too difficult to conclude whether the Miocene forms can be distinguished from the foeda as an independent species or should be included in an infraspecific rank.» AMANO, K., K. TANAKA & S. KARASAWA. 1991. Chlamys (Chlamys) foeda (Yokoyama) from the Shiragami Formation in Nagano Prefecture, Central Japan. Bulletin of Joetsu University of Education, 10 (2): 305-313, pl. 1. [p. 308]
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Chlamys (Chlamys) niikappuensis Masuda; K. Amano, K. Tanaka & S. Karasawa, 1991, Chlamys (Chlamys) foeda (Yokoyama) from the Shiragami Formation in Nagano Prefecture, Central Japan, plate 1, figure 5.
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