Chlamys behringiana plafkeri (MacNeil, 1967)
MACNEIL, F. S. 1967. Cenozoic pectinids of Alaska, Iceland, and other nothern regions. United States Geological Survey Professional Paper, 553: iv, 57 p., pls. 1-25. [p. 32, pl. 12, figs. 1-2, 6-7; pl. 13, figs. 1-2]
1967 Chlamys (Chlamys) pseudislandica plafkeri MacNeil, 1967
F. S. MacNeil, 1967, plate 12.
F. S. MacNeil, 1967, plate 13.
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«Pecten islandicus, MacNeil in Miller, 1953, Jour. Geology, v. 61, no. 1, p. 29.
Description.— Shell moderately large, left valve more inflated than right valve, valves higher than long. Ribs of right valve moderately wide to moderately narrow, or of unequal width, rounded and somewhat crude, entire or divided into two and rarely three parts toward the margin, interspaces opposite primary ribs of left valve wider, deeper, and sometimes with an interstitial riblet. Ribs of left valve weakly divided into primary, secondary, and tertiary ribs, the tertiary ribs numbering one or two between different secondary and primary ribs, higher and sharper than on right valve. Discussion.— Specimens from Middleton Island referred to this subspecies are larger and more rounded than specimens of C. cf. C. hanaishiensis from supposed Pliocene beds on Tugidak Island. The specimens of C. cf. C. hanishiensis from Pliocene beds in the Lituya district (pl. 10, figs. 6, 9-11) are also smaller than C. p. plafkeri and some of the specimens are lopsided. If this lopsidedness proves to be a characteristic that is not due to distortion, this variant is in need of a name. They occur both above and below other beds containing Clinocardium meekianum (Gabb), a species believed to be restricted to the Pliocene. C. hanaishiensis amchitkana, occurs with C. p. plafkeri on Middleton Island, but so far only the former has been found on Amchitka Island in the Aleutians. C. h. amchitkana has narrower ribs, and they have more of a tendency to split toward the margin; the ribs of the left valve are all nearly the same size. C. p. plafkeri appears abruptly in the Middleton Island section; its first occurrence is in a bed immediately above the highest bed, USGS 17746, containing C. (Leochlamys) tugidakensis. Miller's (1953, p. 23) stratigraphic section for Middleton Island shows that within some 3,675 feet of glacial marine sediments there are numerous beds of cobble-sized conglomerate. The conglomerate in which C. p. plafkeri first appears, USGS 17745, is the only one of these conglomerates, or the only part of the section for that matter, that marks a change in the pectinid fauna. The top of the range of C. (L.) tugidakensis is taken tentatively as the top of the Pliocene. Typical C. p. plafkeri is taken to mark the beginning of the Pleistocene. C. coatsi middletonensis, which appears for the first time with C. p. plafkeri, is also known from supposed Pleistocene beds on Amchitka Island. The more coarsely ribbed variants of C. p. plafkeri approach the less coarsely ribbed variants of C. coatsi middletonensis. Possibly not enough specimens are at hand to show beyond all question that these two forms do not merge, but I do not believe they do. C. pilicaensis Kubota (see Masuda, 1962b, pl. 20, figs. 1, 2), described from the Setana Formation (early Pliocene) of Hokkaido, is intermediate bewteen these two forms, however, and they are either descended from C. pilicaensis or all three forms descended from a common immediate ancestor. Masuda said that C. pilicaensis still lives in the northern Pacific; this is probably an allusion to C. beringiana or the subspecies O. b. graui. It must be recognized that the abrupt appearance of C. pseudislandica plafkeri and C. coatsi middletonensis in the Middleton Island section is due to a migration from elsewhere. While it is possible that these are Atlantic migrants, I believe they are more like preexisting Japanese species than any known forms in northern Atlantic or European beds. Types: The holotype (USNM 645002) has a height of 88 mm and a length of 81 mm. A large incomplete unfigured specimen has a height of 100 mm. Other figured specimens are numbered USNM 645003-645006. Type locality: Conglomeratic sandy mudstone 2,185-2,695 ft above base of a 3,675-ft section, near north end of Middleton Island, Alaska, USGS 17745 (for description of section, see Miller, 1953, p. 23). Other occurrences: Middleton Island, same bed as above and higher beds in section, USGS 17743, M1742, M1743, M2054.» FRANCIS STEARNS MACNEIL, 1967
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