Mizuhopecten hashimotoi (Akiyama, 1962)
AKIYAMA, M. 1962. Studies on the phylogeny of
Patinopecten in Japan. Science Reports of the Tokyo Kyoiku Daigaku [Section C -
Geology, Mineralogy and Geography], 8 (74): 63-122, pls. 1-8, text-figs. 1-3. [p. 94, pl. 2, figs. 1, 5]
1962 Patinopecten (Patinopecten) hashimotoi Akiyama, 1962
M. Akiyama, 1962, plate 2.
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«Shell medium in size and thickness, inequivalved, equilateral except for auricles, height as equal as length, nearly straight in side sloping, left valve as convex as the right in the young stage but tending to be flat in the adult; apical angle about 100 degrees.
Right valve convex, provided with 16 somewhat broad, subequal, flat-topped, rather equarish radials which tend to be divided into two parts by a shallow furrow near the ventral margin; interspaces much narrower than radials themselves; concentric striations fine and inconspicuous; both auricles are broken, but as far as observations can be done, they are furnished with conspicuous radial threads and fine concentric striations: anterior one provided with byssal notch; hinge straight, with conspicuous cardinal crura and resilial pit provided with lateral ridges on both margins. Left valve furnished with 16-17 elevated, round-topped radials; interspaces slightly broader than radials themselves; concentric striations conspicuous; both auricles triangular in form and provided with radial threads and concentric striations; hinge straight, with cardinal crura and resilial pit with shallow furrows on both margins corresponding to the right valve. Remarks. This new species is characterized by the right valve with 14-19 round-topped radials usually bipartite near the ventral margin, which take 16 in the most frequent number of radials. The left valve is characterized by having 15-17 elevated, round-topped radials. The present species is somewhat different from murayamai, but hashimotoi may be a descendant of murayamai from the surface sculpture and their geological occurrences. Described specimens. Nakatombetsu-machi, Esashi-gun, Hokkaido. Nokatombetsu formation. Reg. no. 17015 (holotype), 17016 (paratype). Comparisons. The present species is closely related to P. propatulus from the San Pablo formation in California, North America, but the slight differences are there that propatulus has the right valve with rather flat-topped radials and the left valve with conspicuous intercalaries. The surface characters of hashimotoi suggest that the species may be an allopatric species of propatulus. The present new species is also allied to P. ibaragiensis, P. yamasakii ninohensis and P. yessoensis, but can be distinguished from ibaragiensis by the right valve with narrower radials furnished with shallow furrows and the left valve with interspaces with no numerous intercalaries, from ninohensis by less numerous radials with no numerous intercalaries in the left valve, and from yessoensis by having less numerous radials.» MASAHIKO AKIYAMA, 1962
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