Pecten (Swiftopecten) nanakitaensis Nakamura, 1940
NAKAMURA, M. 1940. An interesting pecten from the Nanakita Beds, Nenosiroisi-mura , Miyagi-gun, Miyagi-ken. Transactions and Proceedings of the Palaeontological Society of Japan, 18: 37-38 (263-264), text-fig. 1 [Reprinted from Jour. Geol. Soc. Japan, Vol. 47, No. 561, 1940].[p. 37 (263), text-fig. 1]
«Pecten (Swiftopecten) nanakitaensis, n. sp.
A single right valve. Shell rather small in size, measuring 41mm in altitude, 38.5 mm in breadth, about 7mm in depth and about 85° in apical angle. Valve compressed, broadly pear-shaped, sculptured with about either prominent ribs; posterior ear broken. Ribs eight in number, roundly elevated, about equal to its interspaces anteriorly, its backs and sides provided with about 5 radial threads, which are marked only on the anterior half of shell, posteriorly becoming weaker and at apical region altogether obliterated; interspaces of main ribs provided with 3–4 radial threads which extend posteriorly to only half of shell length. Main ribs much broader than their shallow interspaces at apical region, gradually becoming nearly equal to the interspaces in breadth anteriorly, and at anteriormost margin, nearly equal. Posterior slope of shell provided with several weak radial threads, the anterior slope also provided with a few, which are nearly obsolete. Hinge-line short, posterior ear broken, the anterior one short, provided with a few threads. Interior features unknown.
Locality: — Dôniwayama, Nenosiroisi-mura, Miyagi-gun, Miyagi-ken. Reg. No. 61334. Horizon: — Lower part of the Nanakita beds. Compared with Pecten crassivenius YOKOYAMA [2], a Miocene shell from near Nanao, Kasima-gun, Isikawa-ken, the present one has a smaller shell, fewer number of radial ribs, and the sculpture of the ribs are entirely different. YOKOYAMA'S species is provided with ribs which are trichotomous and coarsely scaly, while the present one lacks the scaly sculpture. Compared with Pecten (Swiftopecten) swiftii BERNARDI, figured by GRANT aud GALE [1] (including the forms parmeleei DALL, wattsi ARNOLD and kindlei DALL, also figured by GRANT and GALE in the same work), the present one has more delicate sculpture on the ears, a less number of much weaker radial threads, a greater number of prominent ribs, a smaller shell, and smaller ears. Compared with Pecten hastatus strategus DALL, figured by ARNOLD [2] from the Pliocene of Santa Barbara, California, the present one has a less number of radial ribs, less orbicular shell outline, less prominent radial threads at the anterior half of the shell, and weaker sculpture on the posterior half of the shell. Some varietal forms of Pecten cosibensis YOKOYAMA [3] and P. heteroglyptus YOKOYAMA [4] also somewhat resemble the present one, but the characters of sculpture readily remove the present one from YOKOYAMA'S species. Pecten (Swiftopecten?) otutumiensis NOMURA and HATAI [5] from the Otutumi beds of the Tomiya Block in Miyagi-ken, somewhat resembles the present species, but the main ribs in that species bifurcate and they are said to be imbricated, while in the present one the ribs do not bifurcate and the whole surface is not imbricated as in that species. At this place the writer wishes to thank Prof. H. YABE for kindly giving him permission to submit this article for publication. He also extends his thanks to Dr. S. NOMURA and Mr. K. HATAI for their suggestions and help during the preparation of this article.» [2] M. YOKOYAMA: Pliocene Shells from near Nanao, Noto, Imp, Geol. Surv. Japan, Rep. No. 104, p. 6, pI. 6, fig. 1, 1929. [1] U. S. GRANT and H. R. GALE: Catalogue of the Marine Pliocene and Pleistocene Mollusca of California and Adjacent Regions, Mem. San Diego Soc. Nat. Hist., Vol. 1, p. 171, pl. 10, figs. 1-5, 7, 1931. [2] R. ARNOLD: New and Characteristic Species of Fossil Mollusks from the Oil-Bearing Tertiary Formations of Southern California, Proc. U. S. Nat. Mus., Vol. 32, No. 1545, pI. 50, fig. 13, 1907. [3] M. YOKOYAMA: Fossils from the Miura Peninsula and Its Immediate North, Jour. ColI. Sci., Imp. Univ. Tokyo, Vol. 39, Art. 6, p. 156, pI. 13, figs. 7, 8, 1920. [4] M. YOKOYAMA: Fossil Shells from Sado, Jour. Fac. Sci., Imp. Univ. Tokyo, Sec. 2, Vol. 1, Pt 8, p. 304, pI. 33, figs, 1-8, 1926. [5] S. NOMURA and K. HATAI: A List of the Miocen Mollusca and Brachiopoda Collected from the Region Lying North of the Nanakita River in the Vicinity of Sendai, Rikuzen Province, Japan, Saito Ho-on Kai Mus., Res, Bull., No. 13, p. 130, pI. 18, fig. 7, 1937. MANZIRÔ NAKAMURA, 1940
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M. Nakamura, 1940, text figure 1.
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