Chlamys (Chlamys) rubida prerubida 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. 23, pl. 11, figs. 7, 8, 13]
«Chlamys (Chlamys) rubida prerubida MacNeil, n. subsp.
Plate 11, figures 7, 8, 13 Description.— Shell moderately small, left valve more inflated than right valve. Anterior ear of right valve moderately large and broad, byssal notch shallow and angulate, byssal area moderately broad; posterior ear short, posterior margin concave, becoming nearly vertical at the hinge line. Anterior ear of left valve unknown; posterior ear short, posterior margin slanting and weakly concave. Posterior dorsal margins slightly longer than anterior dorsal margins. Dorsal slopes of left valve slightly wider than dorsal slopes of right valve. Young juvenile stage of right valve smooth and weakly undulating, faint radial ribs appearing as growth continues. Adult sculpture consisting of moderately small and closely set ribs. Bibs of right valve slightly larger centrally, some of the central ribs splitting abruptly in adults, terminal ribs finer and weakly beaded or scabrous, interspaces evenly rounded and about, the same width as the ribs, some interspaces having an interstitial riblet. Kibs of left valve finely scabrous, divided into primary, secondary, and tertiary ribs with finer interstitial riblets between them; primary and some smaller ribs double or three parted or irregularly divided. Microsculpture reticulate or metal lathelike and restricted to the interspaces of adult stage of the right valve and to very small patches on the left, valve; not present in juveniles.
Discussion.— The microsculpture of the juvenile and adult stages of this subspecies is similar to that of both C. cf. C. hanaishiensis from the same locality (pl. 16, fig. 8) and Recent specimens of C. rubida. A large suite of specimens might show that C. r. prerubida, C. cf. C. hanaishiensis, and C. cf. C. cosibensis from this locality are all part of one intergrading series. Regardless of this possibility, they are regarded as the roots from which several subsequent species and subspecies were derived. C. r. prerubida has finely scabrous ribs, as does C. rubida, and there is a similar tendency for the ribs of the left valve to be three parted. In both C. r. prerubida and C. rubida, the posterior dorsal margin is slightly longer than the anterior dorsal margin, the opposite of the condition in C. beringiana. In all probability C. r. prerubida is a direct ancestor of C. rubida. While the exact relationship of the existing Pacific species C. rubida (Hinds), C. hastata (Sowerby), and C. h. hericius (Gould) is not fully understood and probably will not be until their Pliocene and Pleistocene history is better known, it seems likely that all of them originated in the Pacific. Arnold (1903, pl. 11, fig. 2) figured a specimen from the early Pleistocene of Deadman Island near San Pedro, Calif., as P. (Chlamys) hericeus. This specimen strongly resembles the fragment of C. r. prerubida shown here (pl. 11, fig. 8). C. hericius, which is regarded as a subspecies of C. hastata by most authors, may be the more archaic form of the species. C. hastata and C. rubida may prove, however, to belong to very distantly related sections of Chlamys. Types: The holotype (USNM 644960), a right valve, has a height of 48 mm and a length of 42.5 mm. Two paratypes, both fragments of left valves, are numbered USNM 644961, 644962. Type locality: Near center of the northwest side of Tugidak Island, Alaska, 320 ft. below the top of a section computed to be 3,520 ft thick, 56º31" N., 154º42" W., USGS M1494.» FRANCIS STEARNS MACNEIL, 1967
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F. S. MacNeil, 1967, plate 11.
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