Parvamussium kimurai (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. 320, pl. 46, figs. 1-4]
1965 Variamussium kimurai Hayami, 1965
I. Hayami, 1965, plate 46.
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«Material.— The holotype is a right external mould (GK. H 6301) collected from the lower part of the Monobegawa group at loc. Hy. 6002, mouth of Okuminotani, southwest of Ryoseki, Nangoku City, Kochi Prefecture (HAYAMI coIl.). Paratypes (GK. H 6302-GK. H 6305) from the type locality (HAYAMI colI.) .
Description.— Shell of medium size for genus, suborbicular, very inequivalve, subequilatenil except for auricles, nearly as long as high or a little higher than long, weakly inflated; right valve distinctly smaller than left; in both valves antero- and postero-dorsal margins slightly concave, forming an apical angle of approximately 100 degrees and obtuse angles with both extremities of rounded ventral margin; auricles of moderate size, well defined; anterior one about twice as large as posterior, subvertically truncated in left valve but separated from body of shell by a distinct byssal notch in right valve; posterior auricle obtusely truncated, with a slightly concave posterior margin; dorsal margins of auricles scarcely rising above hinge-line; auricular crura short but stout in right valve; about seven internal ribs diverge from umbonal area; several short internal ribs of secondary order inserted irregularly between primaries; ribs of two orders similar in strength, not much broadened towards ventral extremities, strongly curved outwards, reaching ventral peripheral area in each valve; surface of shell-body of right valve marked with regular con centric lines which become weaker on anterior and posterior areas; auricles marked with similar concentric lines; left valve, on the contrary, ornamented with radial ribs of three orders, namely, about 13 stout primary ribs, weaker secondaries of about the same number, and numerous faint tertiary threads; primaries and secondaries alternating almost regularly; numerous fine concentric lines crossing radials, forming a delicate network with tertiary threads; auricles marked with numerous radial threads of tertiary strength.
Observations and comparisons.— The essential characters of the two valves are well exhibited on the internal and external moulds. The difference in the dimensions and ornamentation of the two valves is well shown in one of the paratypes (GK. H 6304) which represents the exterior of the smaller right valve and the peripheral part of the larger left valve. The presence of a distinct byssal notch throughout the ontogeny is shown by the growth-lines on the anterior auricle of the holotype.
The presence of characteristic radial ornamentation on the left valve indicates that this species is a member of the genus Variamussium. In fact, all its essential characters are not very different from those of Variamussium habunokawense (KIMURA, 1951) from the Upper Jurassic Torinosu series (TAMURA, 1959a, 1960). Furthermore, V. cf. habunokawense is known to occur in the Berriassian of south Kitakami (HAYAMI, SUGITA and NAGUMO, 1960). In V. habunokawense and the comparable species to it, however, the length and height scarcely exceed 8 mm. and the internal ribs are almost of a single order of prominence. The present species is readily distinguishable from Propeamussium yubarense (YABE and NAGAO, 1928), from the Upper Cretaceous of various areas of Japan, by its unequal and better defined auricles and stout radial ribs on the left valve, although some specimens from the Lower Cretaceous of Japan have been compared with that species. P. yubarense was originally described as a variety of Propeamussium cowperi (WARING) from the Upper Cretaceous of California, but, as suggested by ICHIKAWA and MAEDA (1958), it should be treated as a distinct species of Pseudopalliolum OYAMA, 1944, a subgenus of Propeamussium. The concentric ribs of the holotype is somewhat similar to that of the left valve of Propeamussium awajiense ICHIKAWA and MAEDA, 1958a, from the Upper Cretaceous of Awaji island, but the present species is certainly unrelated to that species, because its internal ribs are more numerous and longer, the left valve posseses much stouter radial ribs and the antero- and postero-dorsal margins are more distinctly concave. It is better to consider that this species is a direct descendant of Jurassic Variamussium rather than the ancestral form of Upper Cretaceous Propeamussium (s. l.). In the presence of short inserted secondary internal ribs it is somewhat similar to some species of Parvamussium SACCO, 1897 (type-species: Pecten duodecim lamellatus BRONN), but such strong ornamentation as seen in this species is quite unknown in that genus. The specific name is dedicated to Prof. Toshio KIMURA of the University of Tokyo, who kindly put his collection at my disposal and informed me the occurrence of the present species. Occurrence.— Aritan (upper Neocomian). Black shale of the lower Monobegawa subgroup at loc. Hy. 6002, mouth of Okuminotani valley, southwest of Ryoseki, Nangoku City, Kochi Prefecture. Besides, a poorly preserved specimen, which is probably conspecific, was collected from the Arita formation at Ys. 8, southeast of Suhara, Yuasa town, Arita County, Wakayama Prefecture (HAYAMI colI.).»
ITARU HAYAMI, 1965
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