Nomurachlamys akitana (Yokoyama, 1926)
YOKOYAMA, M. 1926. Tertiary Mollusca from the oil-fields of Akita. Journal of the Faculty of Science, Imperial University of Tokyo [Section II: Geology, Mineralogy, Geography, Seismology], 1 (9): 377-389, pls. 44, 45. [p. 388, pl. 44, figs. 15-17]
1926 Pecten akitanus Yokoyama, 1926
M. Yokoyama, 1926, plate 44.
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«Shell small, rather thick, compressed, orbicular, somewhat higher than long, radiately ribbed.
Right valve: Ribs about twenty-five in number, flattened, usually broader than interstices, often split into two by a median groove; ears unequal, with radiating riblets, anterior longer and larger than posterior, byssal notch triangular. Left valve: Of about the same convexity as the right valve, ribs also about twenty-five, but with an intercalary in every interspace, occasionally splitting into two like those of the right valve; ears unequal, with posterior larger, radiately ribbed. Frequent. One of the right valves is 10 millim. high, 9.3 millim. long and 2.2 millim. deep; while one of the left is 16.2 millim. high, 14.7 millim. long and 3.3 millim. deep. Fossil occurrence.— Beds B : Kurosawa. Beds F: Kinonezaka.» MATAJIRO YOKOYAMA, 1926
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«Remarks: – Chlamys jordani (Arnold) (1906, p. 114, pI. 44, figs. 1, 1a-b) from the Pliocene and Pleistocene of California is distinguishable from akitana by the larger and rather thinner shell, by being inequivalve and by the narrower and deeply channeled interspaces.
Type locality, Geological formation and Age: – Kinonezaka, Yasawagi, Omori-machi, Hiragagun, Akita Prefecture (Lat. 39º22'08''N., Long. 140º21'08''E.). Sugota formation. Early Miocene. Distribution: – Sugota formation, Akita Prefecture; Shunezaka formation, Yamagata Prefecture; Tanosawa formation, Aomori Prefecture; Shiratori member of Kadonosawa formation, Iwate Prefecture; Kunnui formation, Hokkaido: all Early Miocene in age. Occurrence: – Abundant in the very coarse-grained sandstone of the Sugota formation; few in the conglomeratic very coarse-grained sandstone of the Tanosawa formation; rare in the tuffaceous very coarse-grained sandstone of the Kunnui formation.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series - Geology], 33 (2): 117-238, pls. 18-27. [p. 160]
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«The present specimens which are typical, take the following description.
Shell small, rather thick, suborbicular, subequivalve, equilateral except for auricles and radiately ribbed. Right valve with 23-25 round-topped, radial ribs and concentric growth lines, and rarely with intercalarly threads; finely imbricated; radial ribs usually bifurcate at about half of disc-length and broader than the interspaces; the bifurcated radials subequal in strength near margin. Left valve with 23-25 round-topped, radial ribs, intercalary threads and fine concentric growth lines making surface finely imbricated; radial ribs rarely bifurcate and as wide as or narrower than the interspaces; intercalary threads subequal in strength near margin in adult specimens. Anterior auricle of right valve larger than posterior, with distinct byssal notch, narrow byssal area and distinct ctenolium; auricles sculptured with radial threads and concentric lines by which they appear imbricated. Auricles of left valve subequal and sculptured with distinct radial threads and concentric lines, and also imbricated; anterior with wide and shallow byssal notch. Hinge straight, cardinal crura of right valve rather distinct, and lateral ridges of resilial pit widely open towards the lower. Internal surface rather smooth, but with marginal serration. Remarks.— This species is characterized by the rather thick and small shell which is provided with 23-25 round-topped, dichotomizing radial ribs rarely with intercalary threads on the right valve, and by having 23-25 round-topped, rarely dichotomizing radial ribs with intercalary threads on the left valve. The right and left valves are nearly equal in convexity. Pecten (Chlamys) jordani ARNOLD (1906, p. 114, pl. 44, figs. 1, 1a, 1b) from the Pliocene and Pleistocene formations of California, resembles akitana in having numerous, smooth topped, imbricated, dichotomizing radial ribs. ARNOLD's species is distinguishable from akitana by the larger and rather thinner shell, by being inequivalve and by the narrower and deeply channeled interspaces. Dimensions (in mm).--
Type locality and geological formation.— Road cliff near Kurosawa, Taiheimura, Minami-Akita-gun, Akita Prefecture. Wakimoto formation. Pliocene. (There are no subsequent records of collections of this species from the type locality. The writer feels that Kinonezaka in Yazawagimura, Hiraga-gun, Akita Prefecture (Sugota formation) may be the true type locality and geological horizon of the species.)
Described specimens.— Kitano, Yazawagimura, Hiraga-gun, Akita Prefecture. Coarse grained sandstone of the Sugota formation (Miocene). Collected by Dr. K. ASANO and now preserved in the Department of Geology, College of Education, Tohoku University.» MASUDA, K. 1954. On the Miocene Pectinidae from the Environs of Sendai; part 4, Pecten akitanus Yokoyama and Chlamys nisataiensis Otuka. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 13: 111-116, pl. 12. [pp. 112, 113]
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Chlamys akitana (Yokoyama); K. Masuda, 1954, On the Miocene Pectinidae from the Environs of Sendai, part 4, plate 12, figures 8-17.
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