Entolium sanchuense Hayami, 1965
HAYAMI, I. 1965. Lower Cretaceous marine pelecypods of Japan, part I. Memoirs of the Faculty of Sience, Kyushu University, [Serie D - Geology], 15 (2): 221-349, pls. 27-52. [p. 315, pl. 45 figs. 12, 13; pl. 52, fig. 5]
1965 Entolium sanchuense Hayami, 1965
I. Hayami, 1965, plates 45, 52.
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«Material.— The holotype is a bivalved specimen (GK. H 6291) collected from the Ishido formation at loc. Hy. 4001, Ichinose-bashi, south of Kagahara, Nakazato village, Tano County, Gumma Prefecture (ICHIKAWA and HAYAMI coll.). Paratypes (GK. H 6292, GK. H 6293) from the type locality (ICHIKAWA and HAYAMI coIl.).
Description.— Shell very large for genus, usually exceeding 70 mm. in length and height, fairly inequivalve, subequilateral, orbicular, almost as long as high, weakly inflated; test comparatively thick; left valve slightly more strongly inflated than right; antero- and postero-dorsal margins straight; ventral margin smoothly arcuate; hinge-line proportionally long; apical angle approximately 100 degrees; auricles subequal in size; anterior auricle subvertically truncated, while postero-dorsal angle of posterior auricle is obtuse; dorsal margin of both auricles elevated above hinge-line; no byssal notch; surface ornamented with numerous fine concentric striae numbering about 20 to every 10 mm. near ventral margin and more than 30 on umbonal region; concentrics very weak on auricles; main body and auricles marked with numerous faint radial capillae, which are visible only under cross light; a pair of broad internal crural ridges extend from umbonal region to anteroand postero-ventral peripheries; resilifer triangular, fairly deep.
Observations and comparisons.— The present species is represented by three specimens. The holotype is a bivalved specimen, but its left valve is only partly exposed and the test is preserved only on the anterior part of the right valve. Two paratypes from the type locality are very incomplete, but the external and internal characters of the two valves are well recognized from these three specimens.
In the essential characters this species is somewhat similar to the type species of Entolium, E. demissum (PHILLIPS) (STAESCHE, 1926; DECHASEAUX, 1936a) from the Middle Jurasic of western Europe and some other regions. Its suborbicular outline and fine concentric sculpture are especially similar to those of the latter. Accordingly the present species is definitely referable to typical Entolium. The present species, however, differs from E. demissum and its probable Lower Jurassic projenitors by the more rectangularly truncated anterior auricle and generally more prominent concentric ornament. The hinge-line is slightly longer than in the Jurassic species. The present species is probably unrelated to Entolium orbiculare (SOWERBY, 1817) and allied species to it, because the right valve lacks conspicuous widespaced concentric grooves as seen in those species. Entolium utakokense IMLAY, 1961, from the Albian of north Alaska, is a species more or less related to the present one, since the surface ornamentation of its right valve is fairly similar. The dimensions of the shell of the Alaskan species are also exceptionally large for a Cretaceous Entolium. It seems, however, to differ specifically from the present species in view of the stronger convexity of the left valve, the shorter hinge line and the more prominent umbo. Occurrence.— Aritan (upper Neocomian). Fine grey sandstone of the Ishido formation at loc. Hy. 4001, Ichinose-bashi, south of Kagahara, Nakazato village, Tano County, Gumma Prefecture.»
ITARU HAYAMI, 1965
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