Chlamys chinkopensis Masuda & Sawada, 1961
MASUDA, K. & Y. SAWADA. 1961. Some new Tertiary pectinids from Southwestern Hokkaido, Japan. Japanese Journal of Geology and Geography, 32 (1): 19-29, pl. 4 [p. 21, pl. 4, figs. 6-7]
1961 Chlamys chinkopensis Masuda & Sawada, 1961
K. Masuda & Y. Sawada, 1961,
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«Description:— Shell moderate in size, thickness and convexity, higher than long, somewhat posteriorly contorted, nearly equivalve; valves radiately ribbed and forming an angle of about 90° at apex. Right valve with about 30, closeset, more or less elevated, round-topped, frnely and densely scaled radial ribs, finely scaled intercalary threads, concentric growth lines and obtuse network; radial ribs subequal to each other, broader than their interspaces, sometimes divide into two riblets by a shallow longitudinal furrow near ventral margin; radial ribs in central part of disc sometimes gather obscurely into several fascicular bundles separated by broad valleys; valleys between bundles narrower than bundles themselves, very shallow, with a few, finely and densely scaled intercalary threads; fascicular bundles consist of two or sometimes more radial ribs; interspaces between radial ribs round-bottomed, with a single intercalary thread except for valleys between bundles; auricles moderate in size; anterior auricle larger than posterior, furnished with conspicuous byssal notch and moderate byssal area, sculptured with several, finely scaled radial threads and concentric lines; posterior one similar to anterior in sculpture, though with more radial threads than those of anterior; hinge with conspicuous cardinal crura, rather wide and deep resilial pit with short, distinct lateral ridges and conspicuous ctenolium. Left valve with close-set, finely and densely scaled radial ribs, finely scaled intercalary threads, concentric grorvth lines and obtuse network; radial. ribs unequal in strength corresponding to sculpture of right valve, sometimes divide into two or three or rarely more radial threads towards ventral margin or sometimes remain undivided; submargins sculptured with subordinate, finely scaled, fine radial threads; interspaces between radial ribs with a few, finely and densely scaled intercalary threads
in which primary ones appear on upper part of disc; auricles sculptured with several, finely scaled radial threads, intercalary threads and concentric lines. Interior surface smooth but with fine marginal serration. Dimmensions (in mm.) :-- Comparison and Affinity:-- Chlamys islandica (MÜLLER) (R. ARNOLD, 1906, p. 113, pl. 45, figs. 1, 1a) can be distinguished from the present one by its shell which is subcircular, subequilateral, larger number of square and less prominent radial ribs in the right valve and subequal radial ribs in the left. This species also resembles Chlamys erythrocomata (DALL) (1907, p. 170) but differs by its posteriorly contorted shell, rather larger number of finely and densely scaled, round-topped radial ribs and finely scaled intercalary threads in the right valve and by the characteristics of radial ribs which are rather low and finely and densely scaled in the left valve However, since Chlamys erythrocomata has been described by R. DALL without figure from the Okhotsk Sea, its detailed morphological characters remain unknown. Therefore, to make clear the interrelationships among islandica, erythrocomata, pilicaensis (KUBOTA, 1950, p. 97, pl. 9, figs. 56, 69-71), kinoshitai (KUBOTA, 1950, p. 98) and the present new species further study is needed. Chlamys islandica hindsii (CARPENTER) (R. ARNOLD, 1906, p. 111, pI.43, figs. 1, 1a-b) is also distinguishable from the present one by its rather small shell which is subequilateral, rather large auricles, somewhat smaller number of radial ribs in the right valve and by the characteristics of the radial ribs in the left.
Remarks:-- The present one is characterized by its moderate, posteriorly contorted, subequivalved shell, about 30, rather low, round-topped, subequal, finely and densely scaled radial ribs which sometimes gather obscurely into several fascicular bundles, finely scaled intercalary threads in the right valve and by the left valve having the radial ribs which are usually subequal in strength and sometimes divide into a few riblets, finely and densely scaled intercalary threads. Type locality, Geological formation and Age:— Right river cliff of the Toshibetsu-gawa, about 1.5 km. southeast of Chinkope-tôge, Imagane-machi, Setana-gun, Shiribeshi Province, Hokkaido (Lat. 42°24'54''N., Long. 140°08'5''E.). Setana formation. Pliocene. Distribution:-- Setana formation, Hokkaido; Living off Paramushiru Island, Chisima Islands: Pliocene to Recent. Ocurrence:-- Common in the very coarse-grained sandstone.» KÔICHIRÔ MASUDA & YOSHIO SAWADA, 1961
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Discussion.-- This form is distinguished by its suborbicular, moderately inflated shell. It occurs with and may perfectly intergrade with the forms here identified as C. cf. C. hanaishiensis, C. cf. C. cosibensis, and C. trinitiensis. In Japan C. chinkopensis occurs in the Setana Formation (early Pliocene) in association with C. hanaishiensis Masuda, C. pilicaensis Kubota (see Masuda, 1962b, pi. 20, figs. 1, 2), and C. cosibensis (Yokoyama), forms that appear to be closely interrelated as are the members of the complex on Tugidak Island. Future collecting may show that all the above forms are one or, at the most, two species. Even so, the diversity of the complex and the possibility that it foreshadowed several subsequent species makes it desirable to recognize all the names applied to it, if only to designate varietal forms.
C. cosibensis is the oldest of the Japanese names. Masuda (1959b) made both Pecten turpiculus Yokoyama and P. heteroglyptus Yokoyama subspecies of C. cosibensis. The extreme variants of both of these subspecies are less like typical C. cosibensis morphologically than are all the above-mentioned Setana and Tugidak forms, with the possible exception of C. trinitiensis. No attempt is made here to combine any of the Japanese species, and it should not be done, in my opinion, until the complete range of variation of the Pliocene Chlamys in both Japan and Alaska is better known. In all probability enough names have been proposed already to cover any resolution of the synonymy on the species level. Two forms described from early Pleistocene deposits of California, Pecten (Chlamys) islandicus picoensis Waterfall (1929, p. 83, pl. 5, figs. 2, 4) and Pecten (Chlamys) venturaensis Waterfall (ibid., p. 84, pl. 6, fig. 4) are closely related to both C. chinkopensis and C. hanaishiensis, and it is thought to be desirable from the evolutionary standpoint, and for reasons of priority, to treat some of the younger Japanese species as subspecies or varieties of the previously named California species. The interests of the stratigraphic paleontologist are best served by stressing similarities in variation rather than minute and often individual morphological differences. A judicious use of species, subspecies, and varieties is the best means of showing evolutionary and migrational patterns, and it should be the goal of paleontologists to express these relationships in the nomenclature. MACNEIL,
F. S. 1967. Cenozoic pectinids of Alaska, Iceland, and other nothern
regions. United States Geological Survey Professional Paper, 553: iv +
1-57, pls. 1-25. [p. 28, 29]
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Chlamys (Chlamys) cf. C. (C.) picoensis chinkopensis Masuda and Sawada; F. S. MacNeil, 1967, Cenozoic pectinids of Alaska, plate 11, figures 1-3, 5, 6, 10.
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