Nomurachlamys namigataensis (Ozaki, 1956)
OZAKI, H. 1956. Two new fossils from the Namigata Formation in Okayama Prefecture, Japan. Bulletin of the National Science Museum, Tokyo [New Series], 3 (1): 7-13, pI. 2, text-figs. 1, 2. [p. 7, pl. 2, fig. 4]
1956 Pecten (Chlamys) namigataensis Ozaki, 1956
H. Ozaki, 1956, plate 2.
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«Shell large, compressed, circular, symmetrical except for ears. Surface sculptured by 15 radiating ribs, separated by a little narrower valleys. Ribs elevated, high, rounded, divided by longitudinal furrows on the half way from
beak to anterior margin into three unequal parts, two laterals and broader and more elevated medial one; valleys intervened by an intercalary riblets almost equal in size with the lateral ones of main ribs, and the interspaces between ribs and riblets by a finer one; ribs as well as riblets closely crenulated. Ears unequal in size, anterior one elongated and provided with radial riblets. Dimensions: Height 85 mm, length 80 mm. Depository: Nat. Sci. Mus., coll. cat. no. 4379 (Holotype) Locality: Namigata, Nogami ryo, Ibara city, okayama prefecture. Remarks: The present species closely resembles Pecten (Chlamys) kaneharai YOKOYAMA. from the Miocene of Shiobara, Totigi prefecture, but can easily be distinguished from it by different number of ribs which are counted 15 in the former and 20 or more in the latter.» HIROSHI OZAKI, 1956
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«Discussion.— The present species was originally described from the Namigata Formation and is characterized by its rather large shell with 14–15, highly elevated radial ribs with fine striations with a scaly sculpture on the back, and a large anterior auricle with a deep byssal notch. Matsubara (2002b) thought this species to be a member of the subgenus Leochlamys MacNeil, 1967 (type species: Chlamys (Leochlamys) tugidakensis MacNeil, 1967) in the genus Chlamys [Röding], 1798. However, it is tentatively transferred to the subgenus Nomurachlamys Kurihara and Matsubara in Kurihara, 2010 (type species: Pecten kaneharai Yokoyama, 1926) on the basis of the shell sculpture consisting of subregular, stout radial ribs with a few fine, scaly ribs on the radial ribs, an interstitial rib and shagreen microsculpture, and a large anterior auricle. A single left valve (Fig. 6.2) has eleven highly elevated radial ribs. The numbers are smaller than the typical specimens, including the holotype. However, it is thought to fall within intraspecific variation, because other shell characters including a
large anterior auricle and scaly fine ribs on the radial ribs are identical to Ch. (N.?) namigataensis. Distribution.— Namigata Formation (Ozaki, 1956a, b; this study); Maéjima Formation (Matsubara, 2002b); Iwaki Formation (Nemoto and O’Hara, 2001, as “Chlamys ashiyaensis Nagao” and “Patinopecten sp.”). Late Eocene–Early Oligocene.»
MATSUBARA, T. 2013. Molluscan fauna of the “Miocene" Namigata Formation in the Namigata area, Okayama Prefecture, southwest Japan. Journal of the Geological Society of Japan, 119 (4): 249-266, figs. 1-7. [p. 259]
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Chlamys (Nomurachlamys?)
namigataensis (Ozaki); T. Matsubara, 2013, Molluscan fauna of the “Miocene" Namigata Formation in the Namigata area, Okayama Prefecture, southwest Japan, figures 6.1-6.4. |
«Type specimens.— NSMT reg. no. P1-4379 (Holotype and paratypes). Although Ozaki (1956) noted a specimen registered under this number as the holotype, two unfigured specimens are also registered under the same number. They are paratypes.
Material — MNHAH reg. nos. D1-018944 through D1-018951 (from MJ-2c), and D1-018952 (from MJ-4). Emended diagnosis. — Moderate-sized Chlamys (Leochlamys) with 15 to 19 highly elevated radial ribs; fine radial threads appearing above ribs on ventral half of disc; left valve with an intercalary rib; radial sculptures densely imbricated; anterior auricle large, with a deep byssal notch and strong byssal fasciole on right valve; shell surface sculptured by a shagreen microsculpture. Description. — Shell moderate in size, slightly higher than long, suborbicular, rather thin; apical angle between 85° and 95°; both valves with a shagreen microsculpture; lacking internal rib carinae. Right valve weakly inflated; radial ribs 18 to 19, rather irregular, highly elevated, with three radial striations making ridges; ribs on both dorsal parts finer than those on center, finely imbricated; interspace of ribs rather deep, with or without an intercalary rib; anterior auricle large, sculptured by 6 to 7 fine radial ribs; byssal notch deep; ctenolium several in number, strong; byssal fasciole broad, strongly annulated; posterior auricle about half the length of the anterior one, with 7 to 8 radial ribs, weakly oblique anteriorly; resilifer pit moderate in size, with a weak resilifer tooth on both dorsal flanks; anterior dorsal tooth weak, long. Left valve also weakly inflated; radial ribs 15 to 18, highly elevated, rather regular, with three sharp primary striations; an intercalary rib on each interspace; posterodorsal part sculptured by fine, imbricated, radial striations; secondary radial striations appearing at about 40 mm from beak; striations on radial ribs and internal ribs tending to become imbricated ventralward with growth; anterior auricle sculptured by 10 to 14 fine, imbricated radial ribs; posterior auricle as in right valve. Discussion.— Masuda (1962) considered the present species to be a member of the subgenus Mimachlamys Iredale, 1929, as a result of examination of a single right valve collected from the middle Miocene Kawazu Formation in Shimane Prefecture. However, his specimen has lower radial ribs, a shallower byssal notch and a broader apical angle, and is not referred to the present species. The specimens from the Maéjima Formation have a large anterior auricle, deep byssal notch with strong ctenolium, and distinct shagreen microsculpture. The shagreen microsculpture is one of the diagnostic features of the tribe Chlamidini (Waller, 1993). Thus, the present species is referred to the subgenus Leochlamys MacNeil, 1967. The diagnosis of the present species is emended as above, adding the right valve features. Recently, Nemoto and O'Hara (2001) figured a right valve specimen identified as Chlamys ashiyaensis (Nagao, 1928b) from the upper Eocene or lower Oligocene Iwaki Formation of the Shiramizu Group in the Joban Coalfield, northeast Honshu, Japan. Their figured specimen (Nemoto and O'Hara, 2001, pl. 2, fig. 2) is, however, unmistakably referred to the present species. Pecten (Chlamys) ashiyaensis Nagao, 1928b is distinguished from Chlamys (Leochlamys) namigataensis (Ozaki) by having a more compressed shell with more numerous, lower, more irregular radial ribs lacking dense imbrications. Distribution. — Namigata Formation in Okayama Prefecture, Paleogene; Iwaki Formation of the Shiramizu Group in Fukushima Prefecture, late Eocene or early Oligocene; Maéjima Formation in Okayama Prefecture, Eocene or Oligocene.» MATSUBARA, T. 2002. Molluscan fauna of the "Miocene" Maéjima Formation in Maéjima Island, Okayama Prefecture, southwest Japan. Paleontological Research, 6 (2): 127-145, figs. 5, 6. [p. 139, 140]
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Chlamys (Leochlamys) namigataensis (Ozaki); T. Maatsubara, 2002, Molluscan fauna of the "Miocene" Maéjima Formation, figures 5.13, 5.18 (above); figures 6.12, 6.15 (below)
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