Masudapecten yamasakii yamasakii (Yokoyama, 1925)
YOKOYAMA, M. 1925. Tertiary Mollusca from Shinano and Echigo. Journal of the Faculty of Science, Imperial University of Tokyo [Section II: Geology, Mineralogy, Geography, Seismology], 1 (1): 1-23. [p. 17, pl. 5, figs. 1-2, 4-5]
1925 Pecten yamasakii Yokoyama, 1925
M. Yokoyama, 1925, plate 5.
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«Shell medium-sized, rather thin, compressed, orbicular, height equal or slightly greater than length, inequivalve, right valve slightly more convex than left, radiately ribbed; ribs twenty or a little over, which in the right valve are broad, flat and more or less elevated, broader than interspaces and usually equally or unequally tripartite towards the ventral margin, though they may be bipartite in front and behind, while in the left valve they are much narrower than interspaces, elevated though rounded and usually with a single interstitial riblet, though in rarer cases there may be two or more, especially in grown specimens and towards the lateral margins. Ears radiately costellate, equal in the left valve, unequal in the right in which the anterior one is larger and has a shallow byssal notch below. If we take the height equal to 10, the depth of the right valve is not quite 2.
Very common. The largest example we possess is over 90 millim. in height. Fossil occurrence.— Shigarami and Togakushi. There is an ill preserved specimen from Sakae looking like this species.» MATAJIRO YOKOYAMA, 1925
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«Shell medium in size, rather thin suborbicular in outline, inequivalved, equilateral except for auricles, apical angle 105-110 degrees.
Right valve with 19-22, elevated, flat-topped, radials which usually trifurcate and sometimes bifurcate or quadrifurcate, and with fine concentric lines; radials much broader than interspaces which are sometimes provided with 1-2 interstitial riblets; anterior auricle larger than the posterior and furnished with wide and shallow byssal notch, fine concentric lines and several weak radial threads; posterior auricle truncated behind and provided with fine concentric lines and several weak radial threads. Left valve with 20-22, elevated, round-topped, radial ribs which sometimes bifurcate or trifurcate and with fine concentric lines; upper half of the disc furnished with reticulate sculpture; radials narrower than interspaces which are provided with 1 or 2 interstitial trhreads; sculpture of auricles are similar to that of the right valve. Hinge with conspicuous cardinal crura and wide resilial pit provided with conspicuous ridges on both margins. Remarks. In the young forms or somecimes the adolescent forms of this species, radials both of the right and left valves are undivided, but in the adult forms radials are bipartite or tripartite. The location where radials begin to bifurcate or trifurcate has been measured. The measurements of those location takes 12~51 mm. from the beak in the right valves and 33~78 mm. in the left ones. Comparisons. The present species is much allied to P. healeyi from the Purisima and San Diego formations (Pliocene) of the Pacific coast of North America, but the slight differences are there between the two species that healeyi has larger apical angle and more roof-shaped radials than yamasakii. P. hashimotoi is also allied species, but yamasakii can be distinguished from hashimotoi by having more numerous radials bipartite or tripartite in the right valve and provided with conspicuous intercalaries in the left one. Type locality. Exposure along the River Susobana, Shimosoyama, Shigaramimura, Kami-Minochi-gun, Nagano Prefecture. Upper most horizon of the Arakurayama pyroclastics. Reg. no. 17017 (topotype). Distribution. Arakurayama pyroclastics in Nagano Prefecture and Hiramaru formation in Niigata Prefecture.» 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. 95, 96]
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Patinopecten (Patinopecten) yamasakii yamasakii (Yokoyama); M. Akiyama, 1962, Studies on the phylogeny of Patinopecten in Japan, plate 3, figure 3; plate 6, figue 3.
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