Masudapecten masudai (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. 107, pl. 1, fig. 2; pl. 3, fig. 1]
1962 Patinopecten (Masudapecten) masudai Akiyama, 1962
M. Akiyama, 1962, plates 1, 3.
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Description. Shell rather small, rather thin, orbicular, inequivalved, equilateral except for auricles, slightly higher than long, staight in side sloping, auricles small.
Right valve somewhat convex, provided with 24 very low, broad, flattish, often unequal radials, of which backs are sculptured with much faint, shallow furrows near the ventral margin; interspaces much narrower and shallower; concentric striations fine but distinct; anterior auricle ornamented with conspicuous radial threads and concentric striations and provided with rather deep byssal notch with a few ctenolium; posterior one sculptured with conspicuous radial threads and faint concentric striations; hinge straight, with cardinal crura, conspicuous denticles on its extremity and resilial pit provided with lateral ridges on both margins. Left valve much defferent sculpture from the right, less convex, provided with 23-24 faint, obtuse, line-like radials; interspaces much broader than radials themselves; 3 faint interstitial threads are developed on each interspace near ventral margin; concentric striations also distinct; both auricles nearly equal, triangular, with conspicuous radial threads and faint concentric lines; hinge provided with cardinal crura, conspicuous dentiles on its extremity and resilial pit with shallow furrows on both margins, corresponding to the said ridges of the right. Remarks. MASUDA (1958) redescribed Patinopecten iwasakiensis established by NOMURA (1935), based on two specimens from the Sugota formation, of which the right valve was yielded from Ukibuta, Higashiyuri-mura, and the left one was from 100 m. east of Ukibuta tunnel, Omori-machi, both in Yuri-gun, Akita Prefecture. The writer collected and compared the specimen with intact valves from the former locality and many topotype specimens of P. iwasakiensis and concluded that MASUDA's right valve specimen of "P. iwasakiensis" should be distinguished from the said species. Therefore, the writer will, here, propose a new name in honor of Dr. K. MASUDA of Tohoku University. This new species is much allied to P. plebejus, but can be distinguished therefrom by the right valve with less numerous, unequal radials and with the auricle provided with conspicuous radial and deeper byssal notch and the left valve with 3 intercalaries in each interspace. P. iwasakiensis is also allied species but can be distinguished from the present species by the right valve with radials divided by deeper furrows and with smaller auricles and by the left one with elevated radials and more conspicuous intercalaries of which the middle one is rather strong. P. wakuyaensis is also allied to the present species, but can be distinguishable from the latter with the left valve provided with less numerous and weak intercalaries. Described specimens. Ukibuta, Higashi-Yuri-mura, Yuri-gun. Akita Prefecture. Sugota formation. Reg. no. 17031 (holotype), 17032 (paratype).» MASAHIKO AKIYAMA, 1962
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