Argopecten nicholsi (Gardner, 1926)
GARDNER, J. 1926. The molluscan fauna of the Alum Bluff Group of Florida. Part I. Prionodesmacea and Anomalodesmacea. United States Geological Survey Professional Paper, 142-A: 1-79, pls. 1-15, 1 map. [p. 48, pl. 12, figs. 5-6]
1926 Chlamys (Plagioctenium) nicholsi Gardner, 1926
J. Gardner, 1926, plate 12.
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«Shell rather small, thin but tough, slightly inequivalve, the left valve quite compressed, the right feeblyinflated. Umbonal angle a little more than 90º. Dorsal margins rather short, the posterior slightly more produced than the anterior. Ventral arc including more than 180º. Radials 17 in number in each valve of the type, though commonly not exceeding 15, beginning almost at the very tips of the umbones and gradually increasing in width and elevation; adult ribbing strong and even, the summits of the costals broadly rounded or somewhat flattened and forming obtuse angles with the sides, the intercostals broadly concave and of approximately the same width as the costals; secondary threading ordinarily not developed, but both costal and intercostal areas exhibiting a close and microscopically fine radial lineation. Incremental sculpture very obscure in the right valve and on the costals of the left, the intercostal areas of the left valve evenly but not very closely laminated. Ventral margin scalloped in harmony with the radial sculpture. Interior fluted almost to the umbones. Submargins narrow, steeply sloping, free from ornamentation. Auricles of left valve and the posterior auricle of the right subequal, well differentiated from the submargins, and threaded with six or seven lirae, which become increasingly coarse toward the hinge line and which are rendered more or less scabrous by the overriding concentric lamellae; byssal ear narrower than the others and more coarsely sculptured both radially and concentrically. Byssal notch quite deep. Ctenolium including four or five denticles of moderate prominence; length of hinge line more than half the maximum width of the shell. Right valve flexed sharply over the left. Ligament area very narrow, persisting almost to the extremities of the hinge. Resilial pit very small, trigonal, subumbonal. Cardinal crura quite well developed, the surface shagreened in order that they may hold more firmly. Monomyarian, the single adductor scar rather small, slightly posterior, and placed a little above the median horizontal. Pallial line simple, commonly obscure, distant from the base.
Dimensions: Altitude of right valve, 20.6 millimeters; left valve of another specimen 25.0 millimeters. Latitude of right valve, 21.2 millimeters; left valve, 25.5 millimeters. Cotypes: U. S. Nat. Mus. No. 352453. Type locality: No. 3856, 6 miles west-northwest of Mossyhead, Walton County, Fla. A larger valve from the type locality shows a rather fine threading toward the ventral margin most distinct in the intercostal areas. There is a moderate range of variation in relative dimensions, but the costals may number as few as 15 and as many as 19, In Chlamys sayanus the secondary sculpture is introduced very early in the We of the shell. Chlamys chipolanus is relatively higher as a rule, and the concentric laminae are more crowded and more elevated and equally prominent upon the costal and intercostal areas. Chlamys nicholsi Gardner, n. sp., is probably an ancestral type of the gibbus group, so abundant and diversified in the later Tertiary and Recent faunas. The species is by far the most abundant and widespread member of the genus in the Shoal River fauna. Occurrence: Shoal River formation, localities 3856a, 2645r, 3742a , ?5079a, ?5193p, 9958p, 3748p, 7261r, ?7264r , ?5192r, 9960r , 9957c, 10603a, 5618c, 9959c». JULIA GARDNER, 1926
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«Comparison. Argopecten nicholsi most closely resembles A. choctawhatcheensis, from which samples from the middle of the Shoal River Formation (localities 3 and 4) differ in having generally fewer plicae (range 14-21, means 16.9 to 20.7, compared to range 20-24, means 21.0 and, 22.2, in the samples of A. choctawhatcheensis), a more right-convex profile, and secondary radial costae beginning at heights generally greater than 20 mm compared to a lack of secondary costation in the Choctawhatchee species.
Argopecten nicholsi, found at localities 8 and 9 in the "Cardium beds," considered by Gardner (1928, p.235; also Mansfield, 1932a, p. 11, and Smith, 1941, p. 271) to be an upper zone of the Shoal River Formation, resembles A. choctawhatcheensis even more closely in having a greater number of plicae (20-22, mean 20.6) than does A. nicholsi in the middle of the Shoal River Formation of the type area. In fact, a gradation seems to exist between A. nicholsi and A. choctawhatcheensis through the units considered upper Shoal River and the beds on Vaughan Creek (localities 11 and 12), considered lower Arca Faunizone by Mansfield ( 1935, p.14). Exactly how Argopecten nicholsi differs from an undescribed species (Argopecten,? species b, pl. 2, figs. 1-7,9) found in the Oak Grove Sand (locality 2) near its type locality is unclear because the small number of specimens representing the Oak Grove form are all immature, the largest being less than 20 mm in height. The principal difference appears to be the earlier appearance of secondary radial costae (at heights along curvature between 10 and 15 mm) on both the disk flanks and disk of the Oak Grove species. Compared to individuals in the upper part of the known size range of "Chlamys" chipolana Dall (a species found in the Chipola Formation), A. nicholsi differs in lacking the coarse, projecting, concentric lamellae that appear over the tops of some or all of the plicae of both right and left valves, in lacking fine radial costae on the disk flanks, and in having somewhat higher right-valve disk flanks. Stratigraphic range. Middle Miocene (Shoal River Formation). Geographic distribution and ecology. Argopecten nicholsi is found throughout the limited Shoal River outcrop area in western Florida. Gardner (1926, p.2), referring to the formation as a whole, inferred that the environment contained relatively cool, temperate waters that were rather shallow (based on the number of littoral and intertidal genera). In the absence of more exact ecological information from specific localities, A. nicholsi is inferred to have been ecologically generalized, like A. comparilis, and able to live in both an open-marine environment and in bays and sounds of normal salinity». WALLER, T. R. 1969. The evolution of Argopecten gibbus stock (Mollusca: Bivalvia), with Emphasis on the Tertiary and Quaternary Species of Eastern North America. The Paleontological Society. Memoir 3 [Journal of Paleontology, 43 (5, supplement)]: 125 pp. [p. 58]
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Argopecten nicholsi (Gardner); T. R. Waller, 1969, The evolution of Argopecten gibbus stock, plate 2, figures. 8, 10-16; plate 3, figures 1-8.
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