Chlamys akahirensis Kanno, 1958
KANNO, S. 1958. New Tertiary Mollusca from the Chichibu Basin, Saitama Prefecture, Central Japan. Science Reports of the Tokyo Kyoiku Daigaku [Section C - Geology, Mineralogy and Geography], 6 (55), p. 157-229, pls. 1-7, tables 1-4, text-figs. 1-2. [p. 167, pl. 1, figs. 13a, 13b]
1958 Chlamys akahirensis Kanno, 1958
S. Kanno, 1958, plate 1.
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«Chlamys cf . ashiyaensis NAGAO. WATANABE, ARAI and HAYASHI, 1950, Bull. Chichibu Mus. Nat. Hist., 1, pl. 2, fig. 2.
Shell medium in size for the genus, rather thin, compressed, sub-orbicular, subequivalve, though the left valve is more convex than the right one, nearly equilateral except for the ears, almost as high as long; radiately ribbed on the surface. Right valve flatly convex with about 30 round-topped radial ribs which are separated by deep interspaces as wide as the width of ribs. Main ribs usually bifurcate by shallow and narrow sulci from the lower half of disc length, but on some ribs from the early stage of shell and the others from the later stage; the ribs bifurcated from the early stage appear as 2 main ribs but they lack intercalary riblets I in their interspace and are not dichotomous; 1-2 intercalary riblets appear at about the same time or a little earlier than rib-bifurcation, and stronger towards the ventral margin; they are very weak compared with main ribs; both main ribs and intercalary riblets rather scaly. Left valve with sculpture similar to that of right. Anterior auricle of right valve larger than the posterior, with conspicuous byssal notch and distinct ctenolium, and ornamented with about 6 radial ribs; posterior one short, with its outer angle slightly obtuse, ornamented with 6 finely imbricated radial ribs. Anterior auricle of left valve larger than the posterior and with sculpture similar to that of right valve.
Measurement (in mm.) : Comparison:— As to the general outline, this new species is rather related to Chlamys ashiyaensis (NAGAO) (NAGAO, 1928, p. 39, pI. 8, figs. 2, 17; pI. 9, flgs. 1, 2, 9, 10, 20, 21) from the Paleogene of North Kyushu, but it can be distinguished therefrom by having more distinct radial ribs, more narrower interspaces, finer sulci on the top of ribs, and by lacking tripartite radial ribs. Ch. nisataiensis OTUKA (1934, pp. 612-613, pI. 47, fig. 26) from the Kadonosawa Miocene in Iwate Prefecture, is another related species but it differs from the present one by having one intercalary riblet and by the smaller apical angle.
Remarks:— It is noticeable that the present species closely resembles Miocene species of Ch. nisataiensis with respects of their general characters. This relation seems to be more closer than to the Miocene species than to the Paleogene Ch. ashiyaensis. Localities and geological formation:— Loc. 201, right river side exposure, about 70 meters downstream of the Gôhei-bashi (bridge), Obashira, Chichibu-city (type locality), Reg. Nos. 5619 (holotype), 5618 (paratype); lac. 207, a river side cliff, near a fall, at Nenokami, Hikokubo, Yoshidamachi, Reg. No. 5785; loco 139, a several exposures of a mountain side, at Kaminosawa, Ryokami-mura, Reg. No. 5618. The former two belong to the Nenokami sandstone (greenish, conglomeratic sandstone), Upper Oligocene and the latter to the Ushikubitoge formation (granule conglomerate), Oligocene. Associated fauna at the type locality:— Chlamys aff. swiftii (BERNARDI), Patinopecten chichibuensis KANNO, Lima (Acesta) yagenensis OTUKA, Crassatellites pauxillus (YOKOYAMA). Geographic distribution:— Known only from the present area. Geological range:— Oligocene.» SABURO KANNO, 1958
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