Patinopecten skonunensis 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. 42, pl. 4, fig. 2; pl. 6, fig. 7]
1967 Patinopecten (Mizuhopecten) skonunensis MacNeil, 1967
F. S. MacNeil, 1967, plates 4,6.
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«Description.— Only the right valve of this species is known. Shell of medium size, suborbicular, moderately inflated. Beaks moderately overturned, sloping moderately steeply toward the hinge. Dorsal margins very weakly concave, outer margins evenly curved. Hinge line of medium length and ears of medium size; anterior ear moderately broad with about three faint radial riblets, byssal area swollen and sculptured by growth lines only; posterior ear moderately broad, posterior margin sloping at about 45º, radial sculpture hardly discernible; byssal notch shallow; ears nearly flush with dorsal margins and with no open cleft between the ear and margin. Sculpture consists of about 20 moderately prominent radial ribs of variable width, some of the anterior ribs having a tendency to split; interspaces rounded and slightly narrower than the ribs. The shell of the holotype is slightly worn, but there appears to be no clearly defined microsculpture. Muscle scar of medium size. Area between muscle scar and hinge plate with a moderately heavy coarsely crystalline callus. Auricular crurae moderately strong, swollen distally, and diverging weakly from the hinge line terminally. Ligament pit with strong lateral ridges. No ctenolium.
Discussion.— The ears of this species are appressed to the dorsal margins as in P. caurinus, but in practically all other characters it is more closely related to P. yessoensis. The posterior ear slopes even more than in yessoensis; in caurinus the posterior margin of the ear is nearly vertical. Judging from available figures of Japanese species, P. skonunensis is most closely related to the P. hashimotoi group of Akiyama (1962, p. 115), particularly to the P. yamasakii subgroup. Both P. hashimotoi Akiyama (1962, pl. 2, figs. 1, 5) from the Nakatombetsu Formation (early? Pliocene) of Hokkaido and P. yamasakii ninohensis Masuda (see Akiyama 1962, pl. 5, fig. 2) from the Suenomatsuyama Formation (lower part,, probably late Miocene according to Haitai and Nisiyama, 1952) of northern Honshu resemble this species. Masuda (1963, p. 149), however, assigned these two species to different genera, P. hashimotoi to Mizuhopecten and P. yamasakii to Kotorapecten. Of the American species commonly referred to Patinopecten I am inclined to believe P. healeyi (Arnold) (1906, pl. 37, figs. 1, 2) from the Pliocene of California is most closely related to P. skonunensis. P. healeyi has a shallow byssal notch, the posterior margin of the posterior ear slopes strongly, and the umbonal callus is coarsely crystalline and thick. P. healeyi has two-parted ribs on its right valve, but this division into two ribs is restricted to the anterior ribs of P. skonunensis. The deep byssal notch of P. caurinus indicates that typical Patinopecten is closely related to and probably descended from the older American subgenus Lituyapecten. P. healeyi and P. skonunensis have a shallow byssal notch, and in this respect, at least, they are more like Mizuhopecten. Addicott (1965) identified a Nassarius found with P. skonunensis as N. whitneyi (Trask), a species described from the Briones Sandstone (early late Miocene) of central California and reported from supposed middle Miocene beds in the same area. Holotype: UCLA 39474, an incomplete right valve, measures 72 mm in longest dimension; its complete length would be comparable. Type locality: Skonun Point, about 5 miles east of Masset on the north coast of Graham Island, Queen Charlotte Islands, British Columbia, UCLA 4674. Skonun Formation (probably late Miocene). (See MacKenzie, 1916, p. 73.)» FRANCIS STEARNS MACNEIL, 1967
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«Remarks:— This species was described by MACNEIL based upon an incomplete right valve. However, it has auricular crura which are considered to be a very important shell character of Patinopecten (MASUDA, 1963). This species can not be referred to Mizuhopecten.»
MASUDA, K. 1971. Patinopecten from North America. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 83: 166-178, pls. 19-21. [p. 169]
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