Chlamys trinitiensis McNeil, 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. 20, pl. 8, figs. 6, 10-12]
1967 Chlamys (Chlamys) trinitiensis McNeil, 1967
F. S. MacNeil, 1967, plate 8.
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«Description.— Shell medium small, inflated, strongly plicate, gerontic stage tending to be less plicate and more inflated. Anterior ear of right valve moderately large and broad, terminal border rounded, byssal area of medium width, byssal sinus moderately shallow and angulate, ctenolium well defined; posterior ear of right valve short and narrow, posterior border slanting steeply. Posterior ear of left valve short with steeply sloping border; anterior ear unknown. Umbonal angle of moderate width, dorsal margins straight. Dorsal slopes of left valve and posterior dorsal slope of right valve moderately broad, undercut, and weakly concave; anterior dorsal slope of right valve narrow, forming a wide angle with the ear. Sculpture of right valve consisting of poorly to well-defined plicae which develop in the juvenile stage and tend to be obliterated in the gerontic stage. Left valve with medium to moderately broad, usually smooth plicae corresponidng to the interplicae of the right valve, likewise weak or absent in young juvenile and gerontic stages. Secondary sculpture of right valve consisting of moderately strong raised riblets, three to five on the plicae and four to five in the interplicae; riblets inequisized on some plicae with the larger riblets branching; at the beginning of the gerontic stage the riblets may split abruptly into two to four smaller riblets. Interplicae of left valve with three to six narrow but sharply raised riblets; smooth plicae splitting at beginning of gerontic stage into three to five small riblets. Microsculpture of both valves reticulate or metal lathelike, strongest in the secondary interspaces but covering riblets and plicae
as well. Discussion.— The closest known relative of this species is the form figured by Wood (1851, p. 27, pl. 5, figs. 2d-f) as Pecten tigrinus Muller var. γ. According to Wood, P. tigrinus occurs in both the Coralline Crag (late Pliocene) and the Red Crag (early Pleistocene); the horizon of his figured specimen is not given. Most recent lists of Crag fossils only record P. tigrinus from the Coralline Crag. The number of plicae is the most obvious difference between C. trinitiensis and the form figured by Wood; C. trinitiensis has five plicae on its right valve and four strong plicae (with two weaker terminal plicae) on its left valve; Wood's figure shows four plicae on the right valve and three on the left valve. This arrangement of plicae results in a median plication on the right valve of C. trinitiensis and a median furrow on the right valve of C. tigrinus var. γ. Of the known Japanese species, C. trinitiensis most closely resembles C. cosibensis (Yokoyama). It is less like the Pliocene representatives of C. cosibensis (see Masuda, 1959b, pl. 13, figs. 1-9, 16-18) than the early Miocene form, C. cosibensis hanzawae (ibid., figs. 10-15). None of the Japanese forms exhibit the irregularity in the width of the secondary ribs seen in C. trinitiensis, nor do the secondary ribs show the abrupt splitting of the secondary ribs into several smaller riblets. As matters now stand, a Pacific origin for P. tigrinus var. γ seems most likely. It is of interest to note that the gastropod Molleria costulata (Möller) occurs with C. trinitiensis. This minute but very characteristic gastropod has not been found elsewhere in the Pacific. MacGinitie (1959, p. 81) reported it living at Point Barrow, saying it had not been known previously outside the Atlantic. The occurrence of the species in Pliocene beds in the Gulf of Alaska strongly suggests communication with the Atlantic in Pliocene time. Types: The holotype (USNM 644933) consists of a nearly complete right valve and an incomplete left valve, both belonging to the same individual. The right valve measures 41.5 mm in height and 38.5 mm in length. Two figured specimens are numbered USNM 644934, 644935. Type locality: Near the center of the northwest side of Tugidak Island, Alaska, 320 ft below the top of a section computed to be 3,520 ft thick, 56°31" N., 154°42" W., USGS M1494.» FRANCIS STEARNS MACNEIL, 1967
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