Chlamys tanakai Akiyama, 1958
AKIYAMA, M. 1958. New Chlamys from the Environs of Nagano City, Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 31: 243-247, pI. 36 [p. 243, pl. 36, figs. 1-3]
1958 Chlamys ingeniosa tanakai Akiyama, 1958
1958 Chlamys obesa Akiyama, 1958
1958 Chlamys obesa Akiyama, 1958
M. Akiyama, 1958, plate 36.
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«Shell moderate in size, rather thin, higher than long, inequivalved, both valves somewhat convex, subequilateral except for ears; sides slightly concave above, base rounded. Right valve with 28 round-topped, imbricated radial ribs which are usually divided into two parts by a deep, longitudinal furrow near the umbomal area or undivided at the anterior; interspaces narrower than ribs themselves; intercalary threads usually appear at the upper one-fourth of the disc and are round-topped and imbricated; concentric striations inconspicuous; anterior auricle larger than the posterior, ornamented with 6 radial threads and furnished with deep byssal area: posterior auricle truncated behind, ornamented with 9 radial threads; hinge with 8 ctenoliums, cardinal crura, and deep resilial pit which is provided with lateral ridges on both margins. Left valve with 24-29 elevated, round-topped, imbricated, radial ribs which rarely accompany radial threads; interspaces broader than ribs near the ventral margin; near the umbonal area there appear intercalary threads which are imbricated near the ventral margin; anterior auricle larger than the posterior, provided with numerous radial threads and fine concentric striations; posterior auricle truncated behind, ornamented with the same kind of sculpture as the anterior: hinge corresponding to the right valve.
Measurement in mm.:-- Remarks:— This subspecies is characterized by the right valve which is provided with 21-28. imbricated. radial ribs which usually split into two unequal parts by furrow. The left valve is characterized by 24-29. round-topped. imbricated, radial ribs which sometimes accompany radial threads. An intercalary thread is developed in each interspace of the left valve as well as of the right one. The number of radials varies between 21 and 28 on the right valves.
This new subspecies is named in honor of l\lr. Kunio TANAKA, of Shinsyu University, who helped the writer in his field survey. The present new subspecies is allied to Chlamys ingeniosa (YOKOYAMA) (1929, p. 5, pl. 6, fig. 2) in its surface sculpture, but the former can be distinguished from the latter by the much higher shell with smaller apical angle and thinner hinge provided with more rounded resilial pit. YOKOYAMA's species was based upon a single right valve which was collected by him from the Noto Peninsula, and no subsequent records of its occurrence has been published. This subspecies also resembles Chlamys hastatus hindsii (CARPENTER) (ARNOLD, 1906. p. 110, pl. 43, figs. 3, 3a) living in the Bering Sea as well as along the west coast of North America and also as fossil in California, but it is distinguishable therefrom in having more imbricated ribs and undivided radials of the left valve. This subspecies is related to Chlamys islandicus (MÜLLER) (GRANT and GALE. 1931, p. 161, pl. 11, fig. 1) from the Banks of Newfoundland, which has more numerous and less imbricated radials, and is somewhat related to Chlamys branneri (ARNOLD) (1906, p. 55, pl. 3, figs. 9-11) from the Vaqueros formation of California which has broader furrows on the radials of the right valve and larger posterior ear of the left. Chlamys arakawai (NOMURA) (1935, p. 4, figs. 1-2; MASUDA, 1954, p. 150, pl. 19, figs. 1-6.) from the Moniwa formation is a related species to the present one, but the latter is distinguishable from the former in having fewer radial ribs which bifurcate near the beak and in having a thinner hinge plate. Another related species to the present one is Chlamys hataii MASUDA and AKUTSU (1956, p. 130, pl. 20, figs. 1-9.) from the Nagaoka formation, Tochigi Prefecture, which has less numerous and broader ribs that bifurcate only on the lower half of the disc. Localities and Horizon:— Loc. 2*, Kawashita, Togakushi-mura, Kamiminochigun. Lat. 36°40'14.5''N, Long. 138°5'46''E. Loc. 11*, Shimoniregi. Togakushimura. Lat. 36°40'12.8''N, Long. 138°5'2''E. Ogikubo sandstone and mudstone member. Repository:-- The Geological and Mineralogical Institute, Tokyo University of Education; Reg. Nos. 5334 (Holotype), 5336-5350.» * This locality number is shown by TOMISAWA (1953, p. 559) in his geological map. MASAHIKO AKIYAMA, 1958
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