"Pecten" perplanus byramensis Gardner, 1945
GARDNER, J. 1945. Mollusca of the Tertiary formations of northeastern Mexico. Geological Society of America Memoirs, 11: 1-332, pls. 1-27. [p. 62, pl. 10, figs. 1-4]
1945 Pecten (Pecten) byramensis Gardner, 1945
J. Gardner, 1945, plate 10.
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«1829. — — LESUEUR, Walnut Hills fossil shells, pl. 5, fig. 2?
1834. Pecten poulsoni MORTON, Synopsis of organic remains, p. 59 (part)? 1834. Pecten perplanus MORTON, Synopsis of organic remains, p. 58, pI. 5, fig. 5?; pl. 15, fig. 8. 1898. Pecten (Pecten) Poulsoni Morton. DALL, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, p. 719 (synonymy in part excluded). Not Pecten (Aequipecten) perplanus Morton. DALL, Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, p. 732, 1898. 1917. Pecten poulsoni Morton. HOPKINS, U. S. Geo!. Survey, Bul!. 661-H, p!. 27, figs. 3, 3a. 1922. Pecten poulsoni Morton. COOKE, U. S. Geol. Survey, Prof. Paper 129-E, p. 84 (part). 1933. Pecten perplanus Morton. COOKE, Am. Assoc. ~etro!. Geo!., Bull, vol. 17, p. 1388. 1936. Pecten (Pecten) poulsoni Morton. TUCKER, Am. Mid. Nat., vol. 17, p. 476 (part), pl. 1, figs. 5, 6. "Orbicular, somewhat flattened, with about twenty small, simple costae, transversely striated.
"Diameter, from three-fourths of an inch to an inch and a quarter. "Occurs with the preceding species (P. anatipes Morton)." Morton, 1834. The locality given by Morton for Pecten anatipes is "a mass of Nummulite limestone, from Claiborne, Alabama."
The exact locality from which the types of poulsoni and of perplanus were taken is not known. Many of the figures of Lesueur and of Morton are inadequate reproductions and do not indicate the degree of convexity of the shell nor the ribbing pattern toward the distal extremities. Cooke (1933) was the first to record the close relationship between Pecten perplanus Morton and Pecten poulsoni Morton. "The name P. perplanus was originally applied to the flat valve of P. poulsoni Morton, which is unknown in deposits older than the Vicksburg group, and therefore is a synonym of Pecten poulsoni." The specimens from the Oligocene at U.S.G.S. sta. 13581 (P-25), Rancho Gigante, Escondido, Mendez, Tamaulipas, agree essentially with those figured by Hopkins, 1917, except for the flattening toward the umbones, and are tentatively referred to that species. The right valve illustrated in Hopkins' report is from Pearl River, just above the bridge at Byram, Hinds County, Mississippi; the left is from the Byram marl as it is exposed at Vicksburg, Mississippi. The following descriptive notes are based upon the Byram and Vicksburg specimens as the cotypes. The right valve is inflated and slightly wider than it is high; the left valve is flattened, and the apical angle 115° or more. About 18 ribs spring from the umbones of the right valve. They are narrow, rounded, regular in size and spacing at their inception and though they become increasingly broader and higher they remain simple for more than half the distance from the umbones to the ventral margin. A pronounced resting stage is indicated; dorsal to the break, the ribs are simple except for exceedingly faint and fine lirations upon the crests; ventral to it, a medial secondary follows the crest, and the edges of the ribs tend to be pinched so that at the ventral margin of the disk, in some individuals, there are 3 fairly well defined secondaries upon the summit of each primary. The channels between the radials are of approximately the same width as the radials and are evenly and sharply laminated by the incrementals. The number of radials upon the left valve may be lower by one, the secondary sculpture is more obscure or absent altogether, and the concentric imbrication decidedly sharper. The submargins on both valves are narrow, steep, and barbed by the incrementals. The auricles are of moderate size. There is no sharply defined byssal notch or ctenolium but a slight constriction at the base of the right anterior auricle. The usual number of radials· both on the right and the left anterior and posterior auricles is 5 or 4; they are distinct but by no means prominent and are overridden by the sharp and crowded growth laminae. The dorsal margins of the right valve are turned inward and are slightly higher than those of the left. There is a small trigonal subumbonalligament pit and a marginalligament groove extending the length of the dorsal margins. The two pairs of cardinal crurae as well as the strong radial sculpture ally both Pecten poulsoni and Pecten byramensis with Pecten (Pecten) rather than with Pecten (Euvola) Dall. The single muscle impression is large and is contained, for the most part, within the posterior dorsal quadrant. The ribbing pattern crenulates the margin and is reflected over the entire ventral portion of the inner surface. The specimens figured in the Hopkins' report are designated as the cotypes. DIMENSIONS OF COTYPES: Right valve, height, 29.5 millimeters; width, 31.0 millimeters; convexity, 8.6 millimeters. Left valve of another individual, height, 26.7 millimeters; width, 28.7 millimeters; convexity, 3.9 millimeters. DIMENSIONS OF FIGURED SPECIMENS FROM TAMAULIPAS, MEXICO: Right valve, height, 23.5 millimeters; width, 27.0 millimeters. Left valve of a larger individual, height, 30.0 millimeters; width, 32.2 millimeters. Two left valves which may represent the young of Pecten byramensis are also figured. Larger of the two immature left valves: height, 18.0 millimeters; width, 19.5 millimeters. Smaller of the left valves: height, 15.6 millimeters; width, 15.7 millimeters. TYPE MATERIAL, TWO COTYPES: Right valve, U. S. Nat. Mus. 370818; left valve, U. S. Nat. Mus. 370819. Figured specimens from Tamaulipas, Mexico: Right valve, U. S. Nat. Mus. 495927; left valve, U. S. Nat. Mus. 495927. Immature specimens: Larger left valve, U. S. Nat. Mus. 495928; smaller left valve, U. S. Nat. Mus. 495928. TYPE LOCALITY: Right cotype, U.S.G.S. sta. 6454, Pearl River, just above the bridge at Byram, Hinds County, Mississippi; left cotype, U.S.G.S. Sta. 3729, top of bluff opposite seoond sawmill, slightly above second horizon, Vicksburg, Mississippi. Byram marl (Vicksburg group). Adult figured material: Right and left valves, U.S.G.S. sta. 13581 (P-25). Immature figured specimens, U.S.G.S. sta. 13535 (N-17). Pecten byramensis differs from Pecten poulsoni in the lower convexity of the right valve, the higher rib count, and the development of a: secondary liration on the adult primaries. Because of these constant differences and its later appearance in Vicksburg time, Pecten byramensis is considered clo§ely related to Pecten poulsoni but distinct from it. DISTRIBUTION: Upper Oligocene sandstone: ?U.S.G.S. sta. 13581 (P-25); and ?U.S.G.S. sta. 14034 (P-25). Fragmentary examples from U.S.G.S. sta. 13535 (N-17) and from U.S.G.S. sta. 13532 (0-19) are closely related if not identical.» JULIA GARDNER, 1945
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