Patinopecten (Patinopecten) tokyoensis sematensis 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. [p. 93, pl. 8, figs. 1, 2]
«Patinopecten (Patinopecten) tokyoensis sematensis
AKIYAMA, subsp. nov. Pl. 8, Figs. 1 and 2. Shell large in size, thin, suborbicular in outline, inequivalved, equilateral except for auricles; apical angle 100-110 degrees. Right valve with 7 low, flat-topped radials which are divided into 3-5 riblets by rather deep furrows; interspaces narrower than ribs themselves, sometimes with a few interstitial riblets which are difficult to be distinguished from those on the backs of radials; concentric striations weak but distinct; anterior auricle with concentric striations and faint radial threads and slightly folded corresponding to those of the right valve; hinge with conspicuous cardinal crura and resilial pit provided with lateral furrows on both margins.
Left valve with 6-7 narrow, rather roof-shaped radials which are low and obscure towards the ventral margin; a weak radial thread near both submargins of the umbilical part, which disappears near ventral margin; interspaces much broader than radials; the disc furnished with reticulate sculpture on one thirds of the shell from the beak and with fine, inconspicuous threads on the rest of it; concentric striations weak but distinct; anterior auricle with concentric striations and faint radial threads and slightly folded corresponding to those of the right valve. Hinge with conspicuous cardinal crura and resilial pit provided with lateral furrows on both margins. Remarks. This new subspecies is characterized with less convex right valve provided with 7 low radials which are divided into 3-5 riblets. The left valve is characterized by 8 faint radials at the young stage of the shell but with 6 at the adult stage. It is noteworthy that the present subspecies yielded from coarse grained sediments takes the value from 16-21 degrees in curvature index, while tokyoensis from fine grained sediments has the larger value from 22.5-28.5 degrees. Described specimens. Cliff near Sematashinden, Ichihara-gun, Chiba Prefecture. Semata formation. Reg. no. 17012 (holotype), 17013 (paratype). Comparisons. The present new subspecies is much allied to P. tokyoensis tokyoensis, but can be distinguished by the less convex right valve with low, flat-topped radials and with folded byssal area and by the left valve with less elevated radials. Distribution. Semata formation in Chiba Prefecture; Ninomiya formation in Kanagawa Prefecture.» MASAHIKO AKIYAMA, 1962
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M. Akiyama, 1962, plate 8.
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