Nomurachlamys nisataienis Otuka, 1934
OTUKA, Y. 1934. Tertiary structures of the Northwestern End of the Kitakami Mountainland, Iwate Prefecture, Japan. Bulletin of the Earthquake Research Institute, Tokyo Imperial University, 12 (3): 566-638, pls. 47-51. [p. 612, pl. 47, fig. 26]
Y. Otuka, 1934, plate 47.
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«Shell moderate in size, the height almost equal to the length, subequivalve and subequilateral except for the ears, rather thin, compressed, radially costate. Beaks small, sharply pointed, approximate. Ears unequal, the anterior large, triangular, that of the right valve with a deep byssal notch; the posterior triangular, truncated behind at right angles or obtuse. Discs right angled above with the dorsal margins descending slightly concave meeting with the regularly rounded ventral margin at broad angles. Left valve more or less inflated than the right which margin is crenate. Sculpture: anterior ear of left valve with about 6-12 unequal scaly radial riblets (12 in the paratype of the left valve), that of the right valve divided into the upper area with flexuose, Iamellated, horizontal, incremental lines; posterior ears with 6 to 12 low scarbrous riblets, which interspaces with a feeble riblet; the riblet obsolete near the upper margin; discs with 30 to 33 subequal, equidistant, flattopped, roundly edged ribs which are dichotomous at its ventral part (they show the dichotomous character after reaching a height of about 30mm.); interspaces as wide as or a little narrower than the ribs, usually with an intercalating riblet; except for the posterior and anterior part. The posterior and anterior part densely ribbed, and the intercalating riblet obsolete. Incremental lines very fine, not much imbricated. Interior side obscurely radially grooved. Ventral margin strongly crenated.
Measurements:
Height Length Number of ribs 33.5 mm 33.0 mm. 30 (holotype; right valve) -- 55.0 mm. 36? (paratype; left valve) 15.5 mm 15.5 mm. 31 (paratype; right valve) 10.1 mm 10.1 mm. 30 (paratype; right valve) Occurrence: —LOW KAD. SER. Nisatai (loc. 3; rg. no. 1334, paratype and cotypes), Tate (loc. 6; rg. no. 1291 Siratori (loc. 7, rg. no. 1335), Takaba (loc. 9, rg. no. 1336).
The number of ribs are more or less variable in this species. Recently Prof. M. Yokoyama described Chlamys akitana (Yokoyama from the lower horizon of the Akita oil bearing Tertiary, which is most allied to this new species. But the former has a broader interspace between the ribs, fewer ribs, and a straighter antero- and postero-dorsal margin, and more acute angles between the postero-dorsal and the antero-dorsal margin. The young form of Chlamys akitana (Yokoyama) is hardly distinguishable from that of this species. Chlamys meisensis Makiyama [130] is another allied species. But the former has broader interspaces between the ribs, fewer ribs, and a straighter antero- and postero-dorsal margin.» 130) J. MAKIYAMA, op. cit. (1936), 156-157, PI. XIII, Fig. 4.
YANOSUKE OTUKA, 1934
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«COMPARISON WITH FOSSIL SPECIES:— Chlamys akitana (Yokayama) is close to the present species, but the former can be distinguished from the latter by rather thick shell, subequal convexity of both valves, fewer number of radial ribs, and rare bifurcation of radial ribs on left valve. Chlamys jordani (Arnold) is distinguishable from C. nisataiensis by larger shell, much larger anterior auricle and deeply channeled and narrow interspaces. Cryptopecten yanagawaensis (Nomura and Zinbo) resembles C. nisatainensis but it is distingushable therefrom in smaller number and nonbifurcation of radial ribs, and more convex right valve than left one. Chlamys hataii Masuda and Akutsu also closely resembles the present species, but the former is distinguishable from the latter by its smaller number of primary radial ribs, longer cardinal crura reaching to anterior and posterior ends, larger anterior auricles, obscure triangular plane in right valve and larger value of H/W in adult stage. According to Masuda (1962a) C. itoigawae is distinguishable from the present species by the following characters: rather larger shell, radial ribs bifurcating at lower part of disc, intercalary threads appearing on lower part of disc, smaller auricles, posterior auricle truncated behind at obtuse angle, rather narrow byssal area on right valve, primary ribs narrower than interspaces and bifurcation at lower part of disc, and a single intercalary thread which appears simultaneously with or a little earlier than of multiplication of ribs on left valve. These morphological characters are, however also observed in samples of the present species from the Moniwa Formation. C. itoigawae is considered to be conspecific with the present species. Kanno (1960) mentioned that Chlamys ashiyaensis (Nagao) is ancestral to the present species. But it is distinguishable from the present species in having much flatter right valve and larger shell and secondary bifurcation of radial ribs.
COMPARISON WITH RECENT SPECIES:— Among the living pectinids there is a few close allies to the present species. Cryptopecten vesiculosus (Dunker) resembles the present species, but the former differs from the latter in having smaller number of radial ribs which are not separated into two parts by a longitudinal furrow and much more convex left valve. Chlamys islandicus erythrocomatus (Dall) also resembles the present species, but it differs therefrom in having larger shell and smaller number of radial ribs which are divided into three parts by longitudinal furrows. PHYLOGENY:— Phyletic line of the present species is unknown.» SATO, Y. 1991. Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, Northeast Japan and description of their Pectinidae. Report, Geological Survey of Japan, 272: 1-249, pls. 1-34. [p. 48]
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Chlamys nisataiensis Otuka; Y. Sato, 1991, Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, plates 6, 7.
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«Remarks: — Only an imperfect specimen was found in association with the following pectinids; Chlamys sp. Chlamys otukae, Chlamys itoigawae and Aequiepecten yanagawaensis. The present specimen is characterized in having larger shell and larger apical angle, slightly concave dorsal margins, about 25 radial ribs which are sometimes divided into two subequal riblets by longitudinal furrow, and few secondary intercalary threads between the ribs, especially on anterior and posterior sides of shell. The present species resembles Chlamys akitana (Yokoyama, 1926) in number of bifurcated radial ribs, but the former is distinguishable from the latter by having much number of radial ribs, also larger shell, narrow interspaces and intercalary threads.
Locality and Formation: — River side cliff of Asano-gawa at Higashi-Ichise, Kanazawa City, Ishikawa Prefecture, Loc. No. Su-1, Sunakozaka Formation.» OGASAWARA, K. 1976. Miocene Mollusca from Ishikawa-Toyama Area, Japan. The Science Reports of the Tōhoku University [2nd. Series -Geology], 46 (2): 33-78, pls. 11-15. [p. 41]
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Chlamys (Chlamys) nisataiensis Otuka, 1934; K. Ogasawara, 1976, Miocene Mollusca from Ishikawa-Toyama Area, Japan, plate 12, figure 11.
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«Remarks.— This species is characterized by the rather thin, small, rather compressed shell which is provided with 25-32 flat-topped, dichotomizing radial ribs, by the intercalary threads and by the right valve being less convex than the left one. The ribs are usually narrower than the interspaces on the upper half of the disc, but rarely the interspaces are narrower than the radial ribs. The relationship between radial ribs and their interspaces is not constant.
Chlamys akitana (YOKOYAMA) described in the earlier pages is very close to the present one, but can be distinguished from nisataiensis by the rather thick shell, subequal convexity of the valves, fewer number of radial ribs, by the radials being broader than their interspaces, and by the rare intercalary threads of the right valve as well as by the rare bifurcation of the left valve. The cardinal crura of this species is more distinct than in akitana. Pecten (Chlamys) jordani ARNOLD, mentioned in earlier pages, is also related to nisataiensis, but is distinguishable by the larger shell, much larger anterior auricle, and by the deeply channeled and narrower interspaces. Geological significance.-- The rocks from where Chlamys akitana (YOKOYAMA) has been collected or reported by authors comprises sandy siltstone as at Kurosawa, Taihei-mura, Minami-Akita-gun, Akita Prefecture, which is said to be the type locality of the species; this is the Wakimoto formation, granule conglomerate as at Kinonezaka, Yazawagi-mura, Hiraga-gun in the same Prefecture; this is the Sugota formation, coarse-grained sandstone as at Kitano, Yazawagi-mura in the same district and Prefecture; this is the Sugota formation, coarse-grained, tuffaceous sandstone as at Kumanodô, Takadate-mura, Natorigun, Miyagi Prefecture; this is the Moniwa formation, and of green tuff as in the Kunnui formation in southwestern Hokkaiko as reported by T. NAGAO and Y. SASSA. Chlamys nisataiensis OTUKA occurs in formations consisting of coarse·grained sandstone as at Nisatai, Nisatai-mura, Ninohe-gun, Iwate Prefecture; this is the Shiratori formation, and of granule conglomerate or of impure limestone as at Goishi and Kita-Akaishi, Oide-mura, Natori-gun, Miyagi Prefecture. The conditions of the sea bottom on which the above mentioned two scallops once lived can be understood from the nature of the rocks which now preserve them. The thermal conditions of the sea at the time of the building of the Moniwa, Sugota, Shiratori and Kunnui formations is judged by the associated fauna of the two scallops, which comprise bivalves, univalves, echinoids and foraminifers of tropical to subtropical aspect. From the available data it is evident that the two scallops lived in the neritic zone upon a bottom free from mud derived from the land and under the influence of very mild thermal conditions. Although the type locality of Chlamys akitana (YOKOYAMA) has been ascribed to the Wakimoto formation at Kurosawa, Taihei-mura, Minami-Akita-gun in Akita Prefecture, it is also evident that there are no subsequent records nor collections of the species from that locality, and that it is common in the Sugota formation and its correlatives. Accordingly, the writer feels that the locality of Kinonezaka, Yazawagi-mura, Hiragagun, Akita Prefecture (Sugota formation) is the true type locality and that the possibility of some mistake cannot be overlooked. If the afore-mentioned statement concerning the type locality of Chlamys akitana (YOKOYAMA) proves to be as the writer now feels, it follows that both Chlamys akitana (YOKOYAMA) and Chlamys nisataiensis OTUKA are restricted in their geological range to early Miocene, or more precisely to Lower and Middle Miocene.» MASUDA,
K. 1954. On the Miocene Pectinidae from the Environs of Sendai; part
4, Pecten akitanus Yokoyama and Chlamys nisataiensis Otuka. Transactions
and Proceedings of the Palaeontological Society of Japan [N. S.], 13:
111-116, pl. 12. [p. 114, 115]
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Chlamys nisataiensis Otuka, 1934; K. Masuda, 1954, On the Miocene Pectinidae from the Environs of Sendai, part 4, Pecten akitanus Yokoyama and Chlamys nisataiensis Otuka, plate 12, figures 1-7.
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