Mizuhopecten kobiyamai (Kamada, 1954)
KAMADA, Y. 1954. Patinopecten kobiyamai, a new Miocene scallop from the Joban Coal-Field, Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 15: 173-178, pI. 23, 1 map. [p. 174, pl. 23, figs. 1-4]
1954 Patinopecten kobiyamai Kamada, 1954
Y. Kamada, 1954, plate 23.
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«Shell moderate in size, averaging about 85 mm in height in adult, rather higher than long, inequivalved, nearly equilateral, right valve somewhat ventricose, the left valve flat to slightly convex, margins smoothly rounded, sides only slightly concave above. Right valve with 5-6 strong, broadly rounded ribs, most elevated at its central part, where ribs are slightly wider than the interspaces, gradually broadening and becoming rather flat-topped ventrally. Of the radial ribs, posteriormost one low and narrow, sometimes splitting into several obsolete! riolets; interspaces round-bottomed and sometimes provided with faint interstitial threads, whole surface of test marked with faint concentric growth lines. Hinge-line straight, slightly shorter than one-half of shell length. Anterior auricle slightly longer than posterior one, surface of both marked with tine growth lines and four or more radial riblets, byssal notch shallow but distinct; posterior auricle similar to anterior one, its extremity rectangular. Left valve with 6 rounded or roughly roof-shaped radial ribs, on the antero- and postero-dorsal margins, the ribs are weaker than the others which become well defined with sharp edges in the younger part of shell, while rounded near ventral margin. Interspaces between the ribs round-bottomed, wider than ribs and usuallY covered with microscopic cross-hatching network and with concentric growth lines more weakly developed than on right valve, moreover with weakly developed, rounded, closely spaced intercalary threads extending to as far as near central part of shell. Auricles similar to those of right valve, except for lacking byssal notch. Apical angle 90°.
Dimensions (in mm) :--
Comparison:— This new species bears some resemblance to Patinopecten kimurai (YOKOYAMA) = P. murayamai (YOKOYAMA), P. kimurai tiganouraensis (YOKOYAMA), P. ugoensis (HATAI and NISIYAMA) and P. subyessoensis (YOKOYAMA), but may be distinguished from them as to be mentioned below. This species may be distinguished from P. kimurai which ranges from the Honya formation of the Yunagaya group up to the lower part
of the Taga group in the Joban coalfield by the different convexity of the right valve, more flattened left valve, less number of radial ribs and more weakly developed radial threads in the interspaces; from P. kimurai tiganouraensis, which is closely allied to the present species, from the Miocene of Shiogama, Miyagi Prefecture, by its smaller size and less degree of the apical angle; from P. ugoensis by having threads in the interspaces, by the asymmetrical convexity of the valves, having microscopic cross-hatched ornamentation on the left valve, and less number of radial ribs; from P. subyessoensis from the Miocene of South Saghalin, by its smaller size, less number of ribs and by the presence of threads in the interspaces. Remarks:— A well-preserved specimen which was collected by Mr. H. KOBIYAMA from the Nakayama formation of Shim-Yamaguchi, Takaku-mura, Iwaki-gun, Fukushima Prefecture and shown in figures 3a and 3b in Plate 23, may be referred to the present new species, although it is smaller in size and of less convexity. The surface of the younger part of the left valve less than 30 mm in size is covered with minute crosshatching network, while the older part of the valve is marked with minor radial threads not only on the ribs but also in the interspaces between the ribs, their number amounting to 90 on the whole valve. The radial threads consist of an imbrication of minute scales. The last-cited characteristics of the radial threads have not been noticed in any other known species of Patinopecten. These features of the radial threads are not observed distinctly on the specimens from lshimori. GRANT aad GALE (1931, p. 188) established a new genus Vertipecten by designating Pecten nevadanus CONRAD as the genotype. They distinguished the new genus from the genus Pecten by the characteristic features of the radial threads with imbricated scales as is seen on Pecten nevadanus CONRAD. In that occasion, they held the view that Vertipecten is in a close relationship with Patinopecten and that "it may be found more convenient to consider Vertipecten merely as the ancestal section of Patinopectem." The present new species is convex in the right valve and is much flattened in the left, while the reverse is the general case in Vertipecten. But the present new species seems to possess the characters of both Patinopecten and Vertipecten and to occupy an intermediate position between them. It may be possible to consider that the present form is a link conneding the two genera just quoted, however, the material at hand seems to be premature for conclusive remarks. Since the characters of the present new species may neither be assigned to Patinopecten nor Vertipecten in strict sense and since the specimens may be referred to no known genus. it seems worthy to establish a new subgeneric or generic name for it. However, owing to that sufficient specimens are required to study the interior features and the stability of its surface characters, the writer withholds erecting a new generic name until such an opportunity arrives. The specific name of this new species is dedicated to Mr. Hajime KOBIYAMA, who kindly directed the writer to the fossil locality and who offered his specimens for study. It may also be added that his father the late Nobuo KOBIYAMA was also interested in collecting fossils and he also collected this new scallop from the Ishimori shell-bed in May of 1932, which is now in the Saito Ho-on Kai Museum, Sendai, Japan (SHM coli. cat. no. 2305 ). Localites and horizons:-- IGPS [abbreviation for Institute of Geology and Paleontology, Sendai, Japan] Joe. no. Fs-23; About 500 m east of Nakayama, Ono-mura, lwaki-gun, Fukushima Prefecture. (Taira). Lat. 37º 06' 16'' N., Long. 140º 54' 51'' E. Type locality. Miocene Kabeya formation. IGPS coll. cat. no. 72963. H. KOBIYAMA and Y. KAMADA coll. IGPS Joe. no. Fs-37; Shima-Yamaguchi, Takaku-mura, Iwaki-gun, Fukushima Prefecture. (Taira). Lat. 37º 00' 42'' N., Long. 140º 55' 27'' E. Miocene Nakayama formation. IGPS coll. cat. no. 72964. H. KOBIYAMA coll. IGPS Joe. no. Fs-36; Kamôri-saku, Kamitakaku, Iino-mura, lwaki·gun, Fukushima Prefecture. (Taira). Lat. 37º 00' 12'' N., Long. 140º 55' 49'' E. Miocene Nakayama formation. IGPS coll. cat. no. 72965. Y. KAMADA coll. Distribution of similar species:— The following distribution is that of specimens closely resembling the present new species but are not identical. 1) West coast of Motoura River, Nishi-Anecha, Hagifuse-mura, Uraga-gun, Hidaka Province, Hokkaido. IGPS coll. cat. no. 50972. K. IGARASHI coll. 2) Shimo-Myôga, Miyakawa·mura, Kuji-gun, lbaragj Prefecture. IGPS coll. cat. no. 28444. T. lZUMITANI coll. 3) Soeguni, Nishiumi-mura, Suzu-gun, Ishikawa Prefecture. IGPS coll. cat. no. 61482. MATSUSHIMA coll. 4) Fujitoge, Tabanematsu-mura, Kanuma-gun, Fukushima Prefecture. Dept. Geol. Coll. Educ. Tôhoku Univ. Reg. no. 325. K. HONDA coll.» YASUHIKO KAMADA, 1954
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