Chlamys coatsi 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. 17, pl. 14, figs. 6, 7; pl. 15, figs. 1, 2, 5-7]
1967 Chlamys ("Chlamys") coatsi MacNeil, 1967
F. S. MacNeil, 1967, plate 14.
F. S. MacNeil, 1967, plate 15.
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«Pecten (Chlamys) beringianus, Stewart and MacNeil in Powers, Coats, and Nelson, 1960, U.S. Geol. Survey Bull. 1028-P, p. 543.
Description.— Shell of medium to moderately large size, rotund, moderately inflated. Anterior ear of right valve large and broad, byssal area broad; byssal sinus moderately broad, moderately deep, and angulate; posterior ear of right valve short, posterior margin sloping and weakly concave. Anterior ear of left valve moderately large and broad, anterior margin weakly sinuous. Dorsal margins of about equal length, straight or very weakly concave. Dorsal slopes moderately broad, those of the left valve broader than those of the right valve, weakly undercut. Sculpture consisting of broad plicae or fascicles centrally and finer riblets near the anterior and posterior margins; plicae nearly smooth in juveniles but breaking up into secondary riblets terminally; interplicae with secondary and tertiary riblets that develop earlier than the riblets on the plicae; plicae broader than interplicae on right valve, interplicae broader than plicae on left valve. Tops of plicae and riblets with concentric growth lines only; interspaces between riblets with reticulate or metal lathelike microsculpture. Discussion.— The typical form of this species occurs with and may intergrade with Chlamys coatsi middletonensis, here described. Typical C. c. middletonensis comes from beds on Middleton Island that are believed at present to be older than the beds of Amchitka containing typical C. coatsi. Typical C. coatsi has not been found on Middleton Island; the only known occurrence of it outside of Amchitka is a single left valve from the Gubik Formation near Wainwright, Alaska, supposedly from a horizon of about middle Pleistocene age. C. coatsi may be related through C. c. middletonensis to C. pilicaensis Kubota (see Masuda, 1962b, pl. 20, figs. 1, 2) from the Setana Formation (early Pliocene) of Hokkaido. C. pilicaensis has more regular ribs, especially on its right valve, but some specimens of C. c. middletonensis approach it in this respect. The more coarsely ribbed variants of C. chaixensis are similar to C. c. middletonensis, but the range of variation in C. chaixensis suggests that it was a stock already divergent from C. coatsi in late Pliocene time and that it was producing variants wholly unlike any of the variants of C. coatsi. If C. coatsi is related to C. pilicaensis, as is suspected, it probably is related in turn to C. chaixensis through some early or middle Miocene species such as C. otukae. C. otukae is fairly closely related to the earliest known member of the C. cosibensis group, C. c. hanzawae, from the early Miocene. Any attempt to determine the origin of C. coatsi at this time is complicated not only by a time and morphological gap in the Pacific, but also by the abrupt appearance in the Yorktown Formation (late Miocene) of Virginia of C. decemnaria (Conrad) (see Gardner, 1943, pl. 5, figs. 1, 2, 6, 7). I am inclined to doubt the descent of C. decemnaria from Pecten (Chlamys) clintonius Say and P. (C.) clintonius rappahannockensis Mansfield, as postulated by Mansfield (1936, p. 184). The last two forms belong to the genus Placopecten. I believe it likely, however, that C. decemnaria, Placopecten clintonius, and Mya arenaria, all of which occur in the Yorktown Formation, are migrants from the Pacific. C. decemnaria appears to be related to C. coatsi, but its long anterior dorsal margin suggests that it may be related to C. beringiana. It is older than the earliest known Pacific members of either species, and it is younger than the possible Japanese early Miocene predecessors C. otukae and C. cosibensis hanzawae. Although late Miocene representatives of the C. coatsi stock have not yet been found in the Pacific region, I will suggest that the stock was present then and that it migrated to the Atlantic in late Miocene time; it gave rise there to C. decemnaria. Pecten aratus Gmelin, a Recent deep-water species from the northern Atlantic, resembles both C. coatsi and C. c. middletonensis. Jensen (1912, pl. 1, figs. 5a-d) figured two specimens from south of Iceland at depths of 268 and 295 fathoms. His figures 5a and b have some ribs nearly as coarse as the large ribs of C. coatsi, whereas his figures 5c and d have finer ribs more nearly resembling those of the small specimen of C. c. middletonensis figured here (pl. 13, fig. 4). Presumably Jensen's two specimens represent the opposite extremes of variation. The anterior dorsal margin of Jensen's specimens is shorter than the posterior margin. In this respect, at least, it compares more with some variants of C. coatsi and with C. trinitiensis. C. beringiana strategus (Oldroyd, 1924b, pl. 41, figs. 1, 2), the only broad-ribbed living Pacific form with which it might be compared, has a longer anterior dorsal margin. I think it likely that C. arata is the only living representative of the C. coatsi stock. Types: The holotype (USNM 644908), a left valve, has a height of 95 mm and a length of 90 mm. The paratype (USNM 644909), a right valve, measures 70 mm in height, 68 mm in length. Other figured specimens are numbered USNM 644910-644914. Type locality: Poorly bedded loose sand and gravel forming a fill about 75 feet thick at the head of South Bight, Amchitka Island, Aleutian Islands, Alaska, USGS D46. Possibly from an upper horizon in the section; see discussion under C. hanaishiensis amchitkana (p. 31). Other occurrences: South Bight, Amchitka Island, USGS 16908a (paratype); Gubik Formation (horizon probably middle Pleistocene), mouth of Kuk River near Wainwright, northern Alaska, USGS M1828.» FRANCIS STEARNS MACNEIL, 1967
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