Argopecten imitata (Weisbord, 1964)
WEISBORD, N. E. 1964. Late Cenozoic Pelecypods from Northern Venezuela. Bulletins of American Paleontology, 45 (204): 1-564, pls. 1-59. [p. 152, pl. 16, figs. 5-10]
1964 Chlamys (Argopecten) imitata Weisbord, 1964
N. E. Weisbord, 1964, plate 16.
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«Shell thin, fairly large, moderately inflated, subequivalve, inequilateral, the valves always oblique to a greater or lesser extent. Dorso-lateral margins low, hardly concave, diverging at an angle of 103 to 109 degrees, the posterior submargin longer than the anterior, the ventral margin well rounded, the rounding posterior to the middle eccentric, the eccentricity commensurate with the obliqueness of the valve. Byssal gape narrow, the ctenolium without dentition. Auricles of right valve unequal, the posterior one triangular, gracefully concave at the margin and slightly concave in plan, the anterior ear subrectangular, slightly convex in plan, with a small but excavated byssal notch. Both ears of the right valve are rolled over at the summit of the hinge, and the hinge line itself is slightly bowed down toward the beak. Externally the anterior ear of the right valve has a flattish to somewhat bulging fasciolar area adjacent to the disk, and above this there are three or four feeble to obsolete low radial ridges which are generally a little more apparent near the beak than distally; crossing the right anterior ear are growth striae, the striae convex upward and pronounced on the fasciole, rather faint near the margin of the ear but stronger inward where they form raised threads over the radial riblets and on the summit of the hinge; on the lower half of the posterior ear there are four to six radial riblets, but the upper half is relatively smooth; on some right valves, including that of the type, the radial riblets are wholly obsolete, and the entire ear is smooth except for the numerous concentric lineations; the concentric striae of the right posterior ear may be fine or coarse, the latter occasionally forming raised threads at the summit of the hinge near the beak. On the left valve the ears are subequal, the anterior one a little more concave at the margin than the posterior; both ears are slightly concave in plan and are also rolled over at the summit of the hinge, although more narrowly so than on the right valve, and the hinge line is straight and sharp; there may be as many as six small radial riblets on each ear of the left valve but these are generally faint and are crossed by numerous fine concentric laminae none of which forms elevated threads on the hinge summit. Umbonal areas compressed, the sides of the disk near the beak moderately sharp, the beaks extending a little beyond the hinge line. Below the umbos, the disk and submargins are evenly convex. External sculpture consisting of radial ribs and concentric laminae. There are 21 to 26 ribs on the disk, the ribs on the right valve low and broad, and with narrow shallow interspaces, the ribs on the right valve relatively high and square above, obtusely triangular toward the base, separated by interspaces a little wider than the ribs themselves. On the submargins radial ribs are generally absent, but on one right valve in the collection the posterior submargin bears three low broad ones. The concentric laminae are sharp and regularly spaced on the upper half of the valve but crowded basalvvard; they are sharp in the intercostal areas of the left valve but have been smoothed off the crest of the ribs of both valves. On the right valve the concentric markings are occasionally bunched as incremental lamellae which cross the ribs and interspaces in equal prominence. Internally, the ligamental grooves are long and narrow, and below them is a shallow cardinal depression on either side of the triangular chondrophore, the depressions scored by fine vertical vermicular grooves. At or below the base of the ears there is a small pustule or two. The internal ribs are paired, and extend far upward toward the beak; at the ventral margin proper the ribs are separated by flutings, and on the right valve of the holotype the termini of the internal ribs form small wedges projecting downward beyond the rim of the margin.
Dimensions.— Holotype, right valve (L331al), height and width 36.9 mm.; left valve (L331a2), height and width 37.1 mm.; thickness of valves atached 19 mm. Paratype (L331b), young right valve, height 17 mm.; width 16.8 mm.; thickness 2.7 mm. Paratype (U331a), right valve, height and width 27.9 mm.; thickness 5.4 mm. Specimen K334a, right valve, height 43.2 mm. Specimen U335a, right valve, height 34.7 mm.; width 36.8 mm.; thickness 11 mm. Type locality. — Playa Grande formation (Catia member), south side of Playa Grande road, about 220 meters west of W-15. Nine specimens including five doublets, four right valves, and fragments of left valves. Other localities.— Playa Grande formation (Catia member), south side of coast road at east end of Catia La Mar. Four specimens including two left valves and two right valves. Comparisons.— This species is allied to various forms of both the east American C. irradians group and the west American C. circularis group. The most closely related perhaps is Pecten eccentricus Gabb (1873, p. 256) (see Maury, 1917, p. 351, pl. 34, fig. 8; and Pilsbry, 1921, pp. 412-413, pl. 40, fig. 12) from the middle Miocene of the Dominican Republic, but among other minor differences, C. eccentrica has deeper interspaces, more prominent ribs, and smaller ears on the right valve. C. demiurgus of authors (Maury, 1925, pp. 237-238, pl. 14, fig. 5; pl. 16, fig. 6; Harris [in] Waring, 1926, p. 109, pl. 20, figs. 3-4; and Rutsch, 1943, pp. 112-113, pl. 3, fig. 5) from the middle and upper Miocene of Trinidad and the middle Miocene of Venezuela ? and Colombia is a much larger and more coarsely ribbed shell than C. imitata, n. sp., and on young specimens of C. demiurgus the right valve is more convex than the left. It may be noted that in Dall's illustration (1898, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, p. 178, pl. 26, fig. 3) of his type C. demiurgus, the ears of the right valve are shown to be entirely wanting in radial riblets whereas in illustrations by the abovementioned authors the ears of the right valve are prominently ribbed. The intensity of the relatively weak auricular ribs on C. imitata, n. sp. varies considerably, and on some specimens they are absent. Pecten levicostatus Toula (1908, p. 713, pl. 26, figs. 4-6) as described by Olsson (1922, pp. 371-372, pl. 16, figs. 5-7) from the Gatun (middle Miocene) formation of the Isthmus of Panama and Costa Rica is usually a little wider than high whereas on C. imitata the height and breadth are nearly the same. On Pecten circularis venezuelanus F. and H. Hodson (1927, pp. 25-26, pl. 14, fig. 6; pl. 15, figs. 2,4,5; pl. 17, fig. 1) from the Miocene and Pliocene of northwestern Venezuela the cardinal depressions below the ligamental grooves are much more pronounced than on C. imitata, the internal ribs of C. c. venezuelana do not continue far within as in C. irnitata, and the external ribs of the right valve are much higher and flatter than on the Cabo Blanco form. The Pleistocene and Recent C. irradians concentrica (Say) (1822, Acad. Nat. Sci. Philadelphia, Jour., ser. 1, vol. 2, p. 259) from the Atlantic coastal plain area of the southern United States is more gibbous than C. imitata, and not so oblique. In western America the Pliocene to Recent C. circularis (Sowerby) and congeners (see Keen, 1958, p. 72, fig. 132) are much more inflated than the Venezuelan C. imitata, n. sp. On the Pliocene C. mendenhalli (Arnold) (1906, pp. 84-85, pl. 25, figs. 2, 2a, 2b) (see Hertlein, 1925, California Acad. Sci., Proc, ser. 4, vol. 14, No. 1, p. 16, pl. 1, fig. 5; and Hanna, 1926, ibid. No. 18, p. 473, pl. 25, figs. 4-5) the intercostal areas on the left valve are narrower than on C. imitata but otherwise the Venezuelan and Californian shells are much alike. On C cristobalensis (Hertlein) (1925, pp. 19-20, pl. 3, figs. 1, 2, 5) from the Pliocene of California the ribs of the right valve are higher and squarer than on C. imitata, and it is a much larger shell. Grant and Gale, (1931, p. 210) report C. cristobalensis as a living shell from the coast of Peru. The Pliocene C. subdola (Hertlein) (1925, pp. 20-21, pl. 5, figs. 2, 4, 7) from California is yet another species allied to C. imitata, but the margin of the left posterior ear is subtruncate whereas on C. imitata it is gracefully sinuate». NORMAN EDWARD WEISBORD, 1964
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