Parvamussium cowperi yubarensis (Yabe & Nagao, 1928)
YABE, H. & T. NAGAO. 1928. Cretaceous fossils from Hokkaido: Annelida, Gastropoda and Lamellibranchiata. Science Reports of the Tohoku University [2nd. Series -Geology], 9 (3): 77-96, pls. 16, 17 [p. 88, pl. 16, figs. 17-19]
1928 Pecten (Propeamussium) cowperi var. yubarensis Yabe & Nagao, 1928
H. Yabe & T. Nagao, 1928, plate 16.
|
Shell very compressed, small, suborbicular, nearly as high as long; subequivalve, almost equilateral, except for the ears; the dorsal third of the disc triangular, with the apical angle about 120°; the remaining part of shell semicircular; antero-dorsal margin almost straight, the posterodorsal faintly concave, scarcely longer than the anterior; posterior end bluntly angulated. Test thin.
Right valve: Posterior ear shorter, with the outer angle slightly obtuse; anterior ear distinctly notched below, with an acute outer angle; both ears ornamented with lines of growth. External surface covered with numerous, fine, equally spaced, concentric lines and numerous very fine radial lines scarcely visible under a lens: inner surface bearing 7 strong, equally spaced round ribs, extending from umbo nearly to the margin and abruptly truncated at their lower ends. Left valve: Ears subequal, with the outer angles slightly obtuse. Outer surface ornamented with numerous, slightly raised, fine radial striae and lines of growth; inner surface bearing strong radial ribs of unknown number. The present specimens closely resemble P. (P.) cowperi WARING² from the Chico Group of California, but have larger ears. The inner ribs of the right valve are 7 in number in ours and 9 in P. cowperi. The type species seems to be variable in shape; of the two specimens of P. cowperi figured by C. A. WARING, one (right valve) is transversely elongated, and the other (left valve) much higher. The Japanese examples, which arc now regarded as a variety of the Californian species, are relatively constant in shape. Localities and geological horizon: The Pankemo-yûbari, a tributary of the Yûbari-gawa, and the Katsura-zawa, the Ôyubari coal-mines, Yûbari-gun, province of Ishikari; the Sanushibe, a tributary of the Mukawa, province of Ibud; the Upper Ammonite Beds.» ² C. A. WARING: "Stratigraphic and Faunal relations of the Martinez to the Chico and Tejon of Southern California." Proc. Calif. Acad. Sci., ser. 4, Vol. VII, p. 63, pl. VII, figs. 1, 2.
HISAKATSU YABE & TAKUMI NAGAO, 1928
|