Chlamys ashiyaensis (Nagao, 1928)
NAGAO, T. 1928. Paleogene Fossils of the Islands of Kyushu, Japan. Part 2. Science Reports of the Tohoku University [2nd. Series -Geology], 12 (1): 11-140, pls. 1-17. [p. 39, pl. 8, figs. 2, 17; pl. 9, figs. 1, 2, 9, 10, 20, 21]
1928 Pecten (Chlamys) ashiyaensis Nagao, 1928 [partim]
1928 Pecten (Chlamys) ashiyaensis var. denselineatus Nagao, 1928
1928 Pecten (Chlamys) ashiyaensis var. denselineatus Nagao, 1928
T. Nagao, 1928, plates 8, 9.
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«Shell large, rather compressed, inequivalve, the left valve being more convex than the right one; nearly equilateral excepting the ears, almost as high as long; the dorsal third of the disk triangular, with the apical angle about 80°-90°, and the remaining part semicircular; antero- and postero-dorsal margins almost equal in inclination) slightly concave, the latter being a little shorter, and passing more evenly into the posterior margin; hinge line nearly one half the length of the shell. Ears rather small. Test thin.
Left valve: disk moderately convex; sculpture consisting of numerous narrow, slightly elevated, rounded radial ribs and shallow grooves in alternation. both ribs and grooves concentrically and finely imbricated, and crossed by lines of growth. Main ribs about 20 in number; besides several narrower ribs near the anterior and posterior ends. The ribs, except near the umbo, divided into three parts by two longitudinal grooves, with the median part much broader and more elevated than the two lateral parts, which are very narrow; grooves between the main ribs shallow, much wider than the ribs, and each bearing almost always a single) narrower intercalary riblet, except near the umbo. Another still finer riblet appears, when the shell attains about 7 cm. in height, in each interspace between the intercalary and main rib. Ears unequal; the anterior one long, triangular, with its outer angle acute; the posterior one very short, its outer angle being obtuse; both ears ornamented with about 6-8 scaly radial ribs, alternated with finer riblets. Right valve: flatly convex; the sculpture essentially equal to that of the left valve, but the ribs generally more irregular in width and prominency, and usually much broader than the interspaces. Main ribs very often dichotomous from the eary stage of the shell and cccassionally later once more; in consequence, each of the two parts of one rib usually narrower and nearer to one another than the corresponding primary ribs of the left valve. A single intercalary riblet usually alternated with two equally broad riblets (the two parts of a primary rib), or sometimes with one broad, simple main rib, or more rarely with three riblets (the three parts of one primary rib) in a later stage of the shell. Each of the two parts of the dichotomous rib sometimes tripartite near the ventral margin; moreover, one still finer riblet inserted between the intercalary one and the main rib. Ears very unequal; the anterior one long, with a convex outer margin, a deep byssal notch and a well developed ctenolium, and ornamented with about 6 imbricated radial ribs; the posterior one short, with its outer angle slightly obtuse, ornamented with about 7 or 8 also finely imbricated radial ribs. Dimensions HEIGHT LENGTH
79 mm 77 mm ca. 100 ? A number of specimens of this species has been obtained from the Ashiya Group of the Chikuhô coal-field. This species is somewhat similar to P. meisensis MAKIYAMA¹ in the sculpture of the right valve, but is different in that of the left valve. P. kaneharai YOK.² from the Neogene of Japan is allied to ours. The right valve of the former is more convex than the left one, and has tripartite ribs; the left valve is ornamented with dichotomous ribs. P. kanellarai is thus just the reverse of ours in sculpture. Moreover, the concentric scales of the ribs are more prominent and coarser in P. kaneharai than in the present one.
P. (C.) porterensis WEAVER³ from the Lower Oligocene of Washington and Vancouver Island, N. America, is also akin to ours, but is higher and has a longer hinge line and a slightly different sculpture. Localities and geological horizons: The Yamaga Beds; Yamaga, Ashiya-machi, Orio, Orio-machi, and Iwaya, Shimago-mura. The Waldta Beds; Wakita, Shimago-mura. All these localities are in Onga-gun, province of Chikuzen. A fragment of Pecten which perhaps belongs to this species was obtained from the Sasebo Group exposed at Sasebo city, province of Bizen.» ¹ J. MAKIYAMA; "Tert. Foss. N. Kankyô-dô." Op. cit., p. 156, PI. XIII, fig. 4.
² M. YOKOYAMA: "Tert. Moll. Shiobara." Journ. Fac. Sci., Imp. Univ. Tokyo, Vol. I, p. 135, Pl. XVIII, fig. 1; Pl. XIX, figs. 1, 2, 5-7. ³ B. L. CLARK: "Pelecypoda from the Marine Oligocene of Western North America." Op. cit., p. 83. PI. XI, figs. 1, 3. B. L. CLARK and R. ARNOLD: "Fauna of the Sooke Formation, Vancouver Island." Op. cit., p. 24, fig. 1. TAKUMI NAGAO, 1928
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«Remarks: – Chlamys (Vertipecten) porterensis Weaver (1942, p. 87, pI. 16, fig. 4) from the Oligocene of Washington, U.S.A. resembles the present species, but differs by its smaller number of and finely scaled radial ribs and by the left valve having the fairly heavy radial ribs.
Type locality, Geological formation and Age: – Sea cliff facing the mouth of the Onga River, about 400 m. northwest of Yamaga, Ashiya-machi, Onga-gun, Fukuoka Prefecture (Lat. 33°53'56"N., Long. 1300 39'54''E.). Yamaga formation. Late Oligocene. Distribution: – Yamaga, Wakita, Kakinoura and Yukiaino formations in northern Kyushu: Late Oligocene in age.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 33 (2): 117-238, pls. 18-27. [p. 162]
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