Placopecten protomollitus (Nomura, 1935)
NOMURA, S. 1935. Miocene Mollusca from the Nisi-Tugaru District, Aomori-ken, Northeast Honsyu, Japan. Saito Ho-on Kai Museum Research Bulletin, 6: 19-81, pls. 2-8. [p. 41, pl. 6, fig. 3]
1935 Pecten (Pecten) protomollitus Nomura, 1935
«Pecten (Pecten) protomollitus Nomura. 1935c, p. 41, pI. 6, fig. 3.
Placopecten protomollitus (Nomura), Masuda, 1953a, pl. 8, fig. 8. Holotype: – SM, Reg. No. 6086. Remarks: – Placopecten clintonius (Say) (Tucker-Rowland, 1938, p. 52, pI. 1, fig. 11), the type species of the genus Placopecten, from the Miocene Yorktown formation of Maryland, can be distinguished from the present species by its larger shell, less prominent byssal notch and more numerous radial threads which rarely divided. The original description was based upon a single imperfect left valve. Usually the outer surface of right valve in the younger stage is rather smooth with hardly recognizable radial threads, but tends to become somewhat distinct with growth. Sometimes the interspaces between radial ribs of the left valve have three, fine intercalary threads of a primary intercalary thread and two secondary intercalaries between radial ribs and primary one in regular interval, as pointed out by Nomura (1935) as the characteristic feature of the present species. Though the present species is usually small in size, several, rather large to moderate fragmental specimens are in the collection from various localities. Type locality, Geological formation and Age: – Hotatebuchi, Hitotsumori, Akaishi-mura, Nishi-Tsugaru-gun, Aomori Prefecture (Lat. 40°40'N., Long. 140°08'08''E.). Tanosawa formation. Early Miocene. Distribution: – Tanosawa formation, Aomori Prefecture; Higashi-Innai formation, Ishikawa Prefecture; Orito formation, Niigata Prefecture; Kurosedani formation, Toyama Prefecture; Nakahara formation, Chiba Prefecture: all Early Miocene in age. Occurrence: – Common in the medium-grained sandstone of the Tanosawa formation; common in the calcareous fine-grained sandstone of the Orito formation; few in the medium-grained sandstone of the Higashi-Innai formation; common in the granule conglomerate of the Kurosedani formation.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series - Geology], 33 (2): 117-238, pls. 18-27. [p. 193, 194]
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Placopecten (s. s.) protomollitus (Nomura); K. Masuda, 1962, Tertiary Pectinidae of Japan, plate 20, figure 3; plate 22, figures 15, 16.
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