Chlamys (Swiftopecten) donmilleri MacNeil, 1967
MACNEIL,
F. S. 1967. Cenozoic pectinids of Alaska, Iceland, and other nothern
regions. United States Geological Survey Professional Paper, 553: iv +
1-57, pls. 1-25. [p. 12, pl. 3, figs. 1, 4, 6]
Chlamys (Swiftopecten) donmilleri MacNeil, n. sp.
Plate 3, figures 1, 4, 6 Description.— Shell of medium size, moderately inflated; hinge of medium length; umbonal angle moderately narrow in juveniles but flaring strongly in adults; shell weakly asymetrical anteroposteriorly, strongly plicate and strongly varicate. Anterior ear of right valve moderately long, byssal area narrow, byssal notch moderately shallow; posterior ear of right valve short with a nearly vertical posterior margin. Ears of left valve unknown. Right valve has four strong medial plicae and an indistinct narrow plication at each end, interspaces about half as wide as plications, deepest between the growth varices; secondary sculpture consisting of low flattened lirations, 9 or 10 on the plications and 5 to 8 in the interspaces, strongest at the edge and on the forward slope of the growth varices. Left valve has five moderately strong plicae and wider interspaces that are more deeply sunken between the varices; secondary sculpture consists of raised lirations that tend to be weakest midway between the varices, although the crest of some varices may be nearly smooth. Interior features not known.
Discussion.— This species and the following doubtfully identified forms from the Alaska Peninsula are the oldest Swiftopecten to be recorded from America. C. (S.) donmilleri occurs in the part of the Yakataga Formation believed to be of middle and late Miocene age, and the Alaska Peninsula forms occur in association with Mytilus middendorffi Grewingk, believed to be a reliable indicator for the middle Miocene. A Swiftopecten of comparable age and probably closely related to C. (S.) donmilleri was figured by Masuda (1959a, pl. 9, fig. 1) as C. (S.) swifti. It comes from the Otsutsumi Formation (middle Miocene; late early Miocene in a twofold division) of Miyagi Prefecture, northern Honshu. A right valve from the same locality was figured by Nomura and Hatai (1937, pl. 18, fig. 6). Nomura and Hatai (1937, p. 130, pl. 18, fig. 7 (described another species from the Otsutsumi Formation as Pecten (Swiftopecten?) otutumiensis, Masuda (1960, pl. 39, fig. 8) figured another specimen from the same locality as Nanaochlamys notoensis otutumiensis. Whether or not this form proves to be a variant of the form assigned to C. (S.) swifti remains to be seen, but I doubt if it has any relationship to Nanaochlamys notoensis. Masuda (1960, pl. 39, figs. 6, 7) also figured two specimens from the Suenomatsuyama Formation (middle and late Miocene according to Masuda; late Miocene and early Pliocene according to Hatai and Nisiyama, 1952, p. 14) as N. notoensis otutumiensis. These are definitely a Swiftopecten, in my opinion, and the left valve strongly resembles a fragment figured here (pl. 3, fig. 2) from middle Miocene beds at Port Moller, Alaska Peninsula. Types: The holotype (USNM 644882), a right valve, measures 69 mm in height and 64 mm in length. The paratype (USNM 644883) and a figured specimen (USNM 644884) are incomplete left valves. Type locality: 1,500 ft above base of Yakataga Formation (horizon probably middle Miocene), south side of White River near foot of glacier, Yakataga district, Alaska, USGS 6694. Other occurrences: Near base of Yakataga Formation (horizon probably middle Miocene), unnamed creek north of and parallel to Yakataga Ridge, Yakataga district, Alaska, USGS 15437; near base of Chaix Hills section of Yakataga Formation (horizon probably upper Miocene), south flank of Chaix Hills near margin of Malaspina Glacier, USGS 17817.» FRANCIS STEARNS MACNEIL, 1967
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F. S. MacNeil, 1967, plate 3.
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