Masudapecten yamasakii ninohensis (Masuda, 1954)
MASUDA, K. 1954. Fossil Pectinidae from Fukoka-machi, Ninohe-gun, Iwate Prefecture. Saito Ho-on Kai Museum Research Bulletin, 23: 11-14. [p. 13, figs. 1-3]
1954 Patinopecten yamasakii ninohensis Masuda, 1954
«Shell medium in size rather thin, suborbicular in outline, inequivalves, equilateral except for auricles; apical angle 95-105 degrees.
Right valve with 18-21 rather round-topped radial ribs which tend to bifurcate near the anterior submargin in some specimens; radials much broader than ribs themselves; auricles sculptured with weak but distinct concentric lines and shallow byssal notch in the anterior one; hinge provided with conspicuous cardinal crura and resilial pit with lateral ridges on both margins. Left valve with 16-20 round-topped radial ribs which are narrower than ribs themselves; each interspace has an interstitial riblets; auricles furnished with fine concentric lines and inconspicuous radial threads; hinge with conspicuous cardinal crura and resilial pit rvith shallow furrows on both margins corresponding to the right. Remarks. P. hashimotoi, P. yamasakii yamasakii and P. yamasakii ninohensis are much related to each other and hashirnotoi has the simplest sculpture on the outer surface of the disc, while yamasakii has the most complex one. The present subspecies is intermediate between hashimotoi and yamasakii. As P. yamasakii is much similar to hashimotoi in the young stage and to ninohensis in the adolescent stage, it seems that the mutational change may be supposed from hashimotoi to yamasakii through ninohensis. But it proves that these are all yielded from the strata of the Kitaura stage. It may be, therefore, interpreted that the above mentioned lineage from hashimotoi to yamasakii through ninohensis indicates the geographic cline from north to south. Comparisons. The present subspecies is much allied to P. hasimotoi, but can be distinguished from the latter by having more numerous radials and a intercalary in each interspace of the left valve. P. yamasakii yamasakii of the young stage is much similar to the present subspecies in sculpture, but can be distinguished therefrom in adult forms by having tripartite or quadripartite radials in the right valve, bipartite or tripartite radials in the left valve and large apical angle. Type locality. Shiratori-zawa in Fukuoka-machi, Ninohe-gun, Iwate Prefecture. Lower most part of the Suenomatsuyama formation. Reg. no. 17018 (topotype).» 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. 96, 97]
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Patinopecten yamasakii ninohensis Masuda; M. Akiyama, 1962, Studies on the phylogeny of Patinopecten in Japan, plate 5, figure 2; plate 6, figure 4 (below).
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