Miyagipecten saromensis Hasimoto & Kanno, 1958
HASIMOTO, W. & S. KANNO. 1958. Molluscan fauna from the Tertiary formation of Chirai, Kamisaroma, Kitamino-Kuni, Hokkaido. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 32: 285-290, pl. 42. [p. 287, pl. 42, figs. 1-5]
1958 Miyagipecten saromensis Hasimoto & Kanno, 1958
W. Hasimoto & S. Kanno, 1958,
plate 42. |
«Shell moderate in size, orbicular, subequivalve, equilateral, much compressed. Right valve smooth, sculptured with very fine concentric lines of growth and indistinct radial threads; auricles subequal in size, sculptured with concentric growth lines and wide and shallow byssal notch; interior surface inaccessable. Left valve ornamented with radial striations and faint concentric growth lines; radial striations numbering about 20, tend to become obsolete on lower half of disc, and associated with an intercalary between them; intercalary radials disappear on lower half of disc in adult shell, while the main radials extend to its base, though they are rather indistinct; radial striations separated by interspaces much broader than their width; auricles subequal, though the anterior one is slightly larger than the posterior, marked with about five radial ribs and closely arranged concentric growth lines; internal surface slightly grooved in accordance with the surface ornamentation. Height, 66 mm.; length, 67 mm.; hinge-length, 25 mm.; depth, ca. 6 mm.; apical angle. 105° measured on a plaster-cast of the holotype (right valve).
Comparison:— Miyagipecten matsumoriensis MASUDA, 1952 (MASUDA, 1952, pp. 252-253, pl. 24. figs. 4-7), is allied the present new species. but the former differs from the latter by having a larger apical angle, closely arranged and larger number of radial ribs, and fainter radial threads on the lower half of the shell of the leit valve, and further, the interspaces between the radial threads in the former are narrower than in the latter. The surface of the right valve of MASUDA'S species is ornamented with closely arranged concentric growth lines while in the present new species they are hardly developed. Remarks:— The distinction between the genus Miyagipecten and Placopecten has been discussed by MASUDA (1952, pp. 251-252). The present new species some what resembles Placopecten akihoensis (MATSUMOTO), (MASUDA, 1952. pp. 250-251, pl. 24, fig. 103), in possession of radial threads on the right valve, though they are hardly developed, and the radial threads on the left which are weakly developed marginally. However, the present specimens are referred to the genus Miyagipecten because the surface sculpture of the right and left valves are different. Locality and Geological Formation:— Loc. 1, a road side cutting at the entrance of the Jyûroku-go valley, Chirai, Saroma-machi, Tokoro-gun, Hokkaido. The Chirai formation, Upper Miocene in a two hold division. Repository:—Geological and Mineralogical Institute, Tokyo University of Education, Reg. Nos. 6195 (holotype), 6196 (paratype).» WATARU HASIMOTO & SABURO KANNO, 1958
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