Chlamys chaixensis 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. 19, pl. 9, figs. 2, 6-10; pl. 10, 4, 5, 7, 8]
1967 Chlamys ("Chlamys") chaixensis MacNeil, 1967
F. S. MacNeil, 1967, plate 9.
F. S. MacNeil, 1967, plate 10.
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«Description.— Shell of medium size, suborbicular, usually longer than high. Hinge of moderate length. Anterior ear of right valve of medium length and moderately broad, byssal sinus moderately deep, narrow, and acute; posterior ear of medium length, posterior margin sloping gently and weakly concave. Anterior ear of left valve moderately small, margin moderately sloping and concave; posterior ear moderately long with a sinuous margin. Umbonal angle moderate to large. Dorsal margins nearly straight to moderately concave, dorsal slopes narrow and weakly undercut. Sculpture variable, ribs ranging from narrow and uniform to moderately broad and irregular, with or without fascicles; both valves may have moderately broad smooth ribs that divide terminally into several smaller riblets. The interspaces have reticulate or metal lathelike microsculpture.
Discussion.— This species occurs with and may intergrade with the form (pl. 10, figs. 1-3) identified as Chlamys lioica (Dall). Some unfigured fragments of the latter have no radial riblets on either valve. The specimens that come closest to connecting these two forms are the ones figured on plate 9, figure 8, and plate 10, figure 1. If these should prove to be one species, C. chaixensis could be treated as a variety of subspecies of C. lioica, the older name. Chlamys cosibensis heteroglypta (see Nomura and Hatai, 1935, pl. 11, figs. 1, 2) may be the closest relative of this species in the Japanese Pliocene. The only other Japanese species that has much resemblance to it is C. otukae Masuda and Sawada (1961, pl. 4, figs. 1-5), described from the Oido Formation (early Miocene) of Miyagi Prefecture, northeastern Honshu. C. otukae may be the ancestral stock of C. chaixensis; at least I can find no other known Japanese Miocene species from which it could be derived. Some specimens of C. coatsi middletonensis are close morphologically to the holotype of C. chaixensis; the latter probably is an extreme variant of the species. The general range of variation of the two forms is somewhat different, however; 110 varietal series similar to that found at the type locality of C. chaixensis has been found elsewhere in Alaska. Of the older Alaskan species, C. chaixensis appears to be most closely related to some of the forms from the middle part of the Tugidak section here referred to C. aff. C. trinitiensis (pl. 8, figs. 2-5, 7-9). In all probability C. chaixensis was derived from some part of the middle Tugidak assemblage. Possibly the nearly smooth ribbed variant shown on plate 8, figure 9, is an end member of the Tugidak series analogous to the more coarsely ribbed variants of C. lioica. Types: The holotype (USNM 644921), a specimen with paired valves, measures 59 mm in height and 60 rnm in length. Two paratypes are numbered USNM 644922, 644923. Other figured specimens are numbered USNM 644924-644929. Type locality: Highest part of the section in the Chaix Hills (horizon probably upper Pliocene), approximately 6,000 ft above a bed containing Chlamys (Leochlamys) tugidakensis. Malaspina district, Alaska, USGS M1875.» FRANCIS STEARNS MACNEIL, 1967
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