Chlamys otukae 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. 19, pl. 4, figs. 1-5]
1961 Chlamys otukae Masuda & Sawada, 1961
K. Masuda & Y. Sawada, 1961,
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Description:— Shell moderately thick, moderately inflated, somewhat higher than long, equilateral except for auricles, subequivalve; both valves radiately ribbed and forming an angle of about 90° at apex. Right valve with 21 to 23, elevated, flatly round-topped, smooth radial ribs, smooth intercalary threads, concentric growth lines and obtuse notwork; radial ribs somewhat broader than their interspaces, divide into two subequal riblets by a shallow longitudinal furrow towards ventral margin or rarely remain rfndivided; intercalary threads usually áppear at about half of disc, usually tend to become broader ventrally and sometimes nearly equal to divided riblets of radial ribs in strength at ventral margin; interspaces between radial ribs usually with obtuse network; submargins with faint, fine radial threads and obtuse network; auricles moderate in size; anterior auricle larger than posterior, furnished with prominent byssal notch and more or less wide byssal area, sculptured with more or less imbricated radial threads and concentric lines; posterior auricle nearly similar to anterior in sculpture; hinge with distinct cardinal crura, rather wide and deep resilial pit with distinct lateral ridges widely open downwards and prominent ctenolium. Left valve nearly equally convex or slightly more inflated than right valve, with sculpture nearly similar to right valve, though radial ribs near posterior submargin sometimes with fine scales. Interior surface of both valves tather smooth but with distinct marginal serration.
Dimensions (in mm.) :-- Comparison and Affinity:— This species can be distinguished from Chlamys hataii MASUDA and AKUTSU (1956) by its small, somewhat less inflated and rather thin shell and smaller proportion of the shell height to hinge-length, but the external sculpture is nearly similar. Chlamys kumanodoensis MASUDA 1953 is distinguishable from otukae by its more number of and very low radial ribs.
Remarks:— This species is named in honor of the late Yonosuke OTUKA of the Geological Institute, Tokyo University, who first figured the present species as Chlamys sp. from the Nanao formation in Ishikawa Prefecture (1935). The present species was also recorded by K. MASUDA and J. AKATSU (1956) as Chlamys cf. hataii from the Oido formation. However, the specimens recorded as Chlamys cf. hataii differ specifically from Chlamys hataii, so that, a new name is proposed. By the re-examination of the type specimen (Reg. No. 10113, Tokyo University of Education) the writers consider that Patinopecten sp. which was described based upon a single inner cast from the Kobana formation in Tochigi Prefecture by K. HIRAYAMA (1954) may be referred to the present species with query. Among the paratype specimens there are some with surface sculpture different from the holotype specimen, that is to say, some radial ribs in the central part of the right valve consist of a set of two radial ribs with no intercalary thread between them, and the interspaces between the two pairs or between a pair and a single radial rib sometimes posses a primary intercalary thread and a faint, fine intercalary thread, so that, in such a case some radial ribs of the left valve are broader and more distinct than others and provided with primary and secondary intercalarly threads. Sometimes the concentric constrictions are distinct. The specimens from the Ōgane formation in Tochigi Prefecture are sometimes rather large in shell size compared with those from the type locality and sometimes the radial ribs of the larger specimens are divided into two riblets at about half of the disc and further into two or rarely three, more or less imbricated radial threads by shallow longitudinal furrows. Type locality, Geological formation and Age:— Road-side exposure at Oido, Motowakuya, Wakuya-machi, Tôda-gun, Miyagi Prefecture (Lat. 38°32'01''N, Long. 141°08'04''E.). Oido formation. Early Miocene. Distribution:— Oido and Utsuno formations, Miyagi Prefecture; Nanao formation, Ishikawa Prefecture; Kobana (?) and Ōgane formations, Tochigi Prefecture; Imagane formation, Hokkaido: Early to Middle Miocene in age. Occurrence:-- Common in the tuffaceous conglomeratic coarse-grained sandstone of the Oido formation; few in the calcareous coarse-grained sandstone of the Nanao formation; common in the calcareous coarse-grained sandstone of the Imagane formation. KÔICHIRÔ MASUDA & YOSHIO SAWADA, 1961
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«Remarks:— The present species is characterized by having about 20 radial ribs, almost all of which are divided into two subequal riblets by a shallow longitudinal furrow from the central part to the ventral margin of disc, and sometimes covered with obtuse network sculpture or with fine transverse lammelae. Auricles are usually sculptured with radial threads. Chlamys hataii Masuda and Akutsu, 1956 resembles the present species, but the former is distinguishable from the latter by having inflated shell, smoother radial ribs and rather simple surface sculptures. The present species has been recorded to date from the Miocene Imagane Formation, Hokkaido; Shiratori Member of Kadonosawa Formation, Iwate Prefecture; Oido and Utsuno formations, Miyagi Prefecture; Kobana and Ogane formations, Tochigi Prefecture; Nanao Formation, Ishikawa Prefecture.
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. 42]
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Chlamys (Chlamys) otukae Masuda and Sawada, 1961; K. Ogasawara, 1976, Miocene Mollusca from Ishikawa-Toyama Area, Japan, plate 12, figures 1, 2.
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