Placopecten clintonius (Say, 1824)
SAY, T. 1824. An account of some of the fossil shells of Maryland. Journal of the Academy of Natural Sciences of Philadelphia, 4 (1): 124-155, pls. 7-13. [p. 135, Pl. 9, fig. 2]
T. Say, 1824, plate 9.
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«P. Clintonius*. PI. ix. fig. 2. Auricles equal; surface with from one hundred and forty to one hundred and eighty elevated longitudinal lines.
DESCRIPTION. Shell suborbicular, compressed, with very numerous, regular, elevated striae, which are muricated with minute scales formed by transverse wrinkles, that are sparse in the middle of the length, and crowded each side of the shell; the intervening spaces are regularly concave, and in parts very distinctly wrinkled: auricles equal, striated like the general surface: within simple, margin striated. Length four inches; breadth rather more. This is a very fine shell, comparable with the magellanica; but the sides below the auricles decline much more rapidly towards the base, and the striae, judging from Bruguiere's figure, are much more prominent and distinct.» * Mr. Finch requested that three species of his collection that might prove to be new, should be dedicated to the distinguished men whose names these shells bear. THOMAS SAY, 1824
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«DlSCUSSION.— Placopecten clintonius, a Miocene species, was considered to be the ancestor of the living scallop, P. magellanicus, by Dall (1898:726) and Gardner (1944:37) because of the close morphologic similarity. The absence of any intermediate forms between these two species is probably due to the absence of fossiliferous cool water deposits of latest Miocene and Pliocene age in the Atlantic Coastal Plain. Beds of this age in the Lee Creek Mine and surroundirrg areas of North Carolina and southeastern Virginia are of a considerably warmer character (Gibson, 1967; Hazel, 1971), which would seem to exclude the P. clintonius stock on environmental grounds. P. clintonius is not found south of central North Carolina (the Lee Creek Mine) in the Miocene, and specimens of the presumed living descendant, P. magellanicus, have their southern boundary in the Atlantic Ocean approximately at Cape Hatteras, North Carolina. Fossiliferous cool-water deposits of latest Miocene and Pliocene age are not found on land in the more northern Atlantic Coastal Plain, but undoubtedly occur on the northern continental shelf and there should contain the intermediate forms.
Dall (1898: 726) mentioned the difficulties in separating the two species, and used the following criteria: The latter [P. magellanicus] can, however, be at once discriminated from the fossil by the shorter hinge-line, higher auricles, much narrower resiliary pit, and usually, the smaller and less central adductor scar of the recent shell... As a rule the radiating threads in the fossils are markedly coarser than those of the living species. The ornamentation, as represented by the radial costae, is generally considerably coarser in P. clintonius, as mentioned by Dall (1898: 726). Specimens of P. magellanicus have finer costae ranging from 10 to 16 per centimeter at 100 mm height. However, the more southern forms of P. magellanicus have sufficiently coarse costae to overlap with some of the more finely costate P. clintonius. It appears that smre P. clintonius have coarser ornamentation than any P. magellanicus, and many P. magellanicus have finer ornamentation than any P. clintonius, but there is overlap in the middle of the sculptural range. A population study of the two species was made by comparing P. clintonius clintonius from the Lee Creek Mine (from spoil bank material derived from the lower two feet (61 cm) of the Yorktown Formation) with living P. magellanicus taken on the Atlantic shelf east southeast of Long Island at 67 fathoms (USFC Sta. 2244). The comparison involved those characters used by Dall and subsequent authors as diagnostic between the two species, and some additional ones. As mentioned by Dall (1898: 726), Tucker-Rowland (1938: 53), and Mongin (1959: 299), the resilifer is considerably broader in P. clintonius than in P. magellarlicus. The gently sloping sides of the resilifer in many specimens make it difficult to obtain an accurate measurement. Because of the possibility that the size and shape of the resilium is not always closely correlated with the resilifer, measurements were made on both for comparative purposes. The ratio of length to height of the resilial insertion differs between the two species, with P. clintonius having a considerably broader resilial insertion (Figure 16), supporting the views of Dall, Tucker-Rowland, and Mongin. The resilifer also varies between the two species, with P. clintonius again having a broader one. While the correlation between height and length of the resilifer is lower than that of the resilial insertion, probably reflecting in large part the difficulty of accurately measuring the resilifer because of the gently sloping sides, this character could be used in species where the resilial insertion is not readily preservable. Valves of P. clintonius clintonius from the Lee Creek beds are posteriorly oblique in shape, whereas valves of P. magellanicus are very slightly, if at all, oblique . This difference is noted by comparing the anterior and posterior half-lengths of the valves of the two species. As indicated in Figure 17, P. clintonius clintonius has a considerably larger posterior half-length, whereas P. magellanicus has nearly equal posterior and anterior half-lengths. Other specimens of P. clintonius clintonius from various localities in Virginia do not exhibit as marked an obliquity of the valves. Several of the other characters mentioned by Dall and others as distinguishing the two species were also studied. Dall (1898: 726) and Tucker-Rowland (1938: 53) mention that a distinguishing character of P. magellanicus is the shorter hinge line as compared to P. clintonius clintonius. ln the populations studied, no difference in length of the hinge line can be seen between the two species (Figure 18). If anything, the larger specimens of P. magellanicus have a generally longer hinge line rather than shorter. Another differentiating character mentioned by Dall (1898:726) is that the auricles are higher in P. magellanicus. No significant difference was noted in this study (Figure 19). Nevertheless the general trend is toward a slightly higher posterior auricle in P. clintonius clintonius rather than otherwise. STRATIGRAPHIC AND GEOGRAPHIC RANGE.— This species occurs in abundance in the lower 2 to 3 feet (0.6 to 0.9 m) of the Yorktown Formation in the Lee Creek Mine. Mansfield (1929b: 1) used this species to name his lowest zone of the Yorktown Formation in Virginia, calling it zone 1 or Pecten clintonius zone. P. clintonius clintonius is restricted to this zone, and is an excellent guide fossil in North Carolina and Virginia. Another subspecies, P. clintonius rappahannockensis (Mansfield) is found in the beds immediately underlying Mansfields zone 1 of the "Yorktown" in North Carolina, and a slight distance below in several localities in Virginia. These beds were placed in the "Virginia St. Marys" beds by Mansfield (1944) but were included in the Yorktown Formation by Gibson (1971).» GIBSON, T. G. 1987. Miocene and Pliocene Pectinidae (Bivalvia) from the Lee Creek Mine and Adjacent Áreas. Smithsonian Contributions to Paleobiology, 61: 31-112, pls. 1-31. [pp. 59, 60]
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Placopecten clintonius clintonius (Say) (= P. clintonius donaldi (Tucket-Rowland)); T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine, holotype of P. c. donaldi, plate 13, figures 1, 3.
Placopecten clintonius clintonius (Say); T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine, plate 12, figures 2, 4 (above); plate 14, figures 1, 2, 5-7 (middle); plate 15, figures 1, 4 (below).
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«Verrill united the Miocene species with the Recent Pecten magellanicus, its undoubted descendant, but Dall, op. cit, 1898, later excluded the Recent species because of "the shorter hinge line, higher auricles, much narrower resiliary pit, and usually the smaller and less central adductor scar of the Recent shell. * * * As a rule the radiating threads in the fossil are markedly coarser than those of the living species. In both, the byssal notch of the adult is represented by a shallow sinuation, and the ctenolium, present in the immature stages, is usually buried in shelly matter in the adult."
The species is well characterized by the thin, compressed shell, the hyaline texture of the interior surface, and the very numerous, irregularly wrinkled, radial lirations of the exterior surface, which are coarser than those of the Recent magellanica. The concentric ornamentation is visible only in the interspaces and on the auricles and submargins, where it is sometimes strong enough to muricate the lirae. It is interesting to note that in the closely allied Recent species the individuals that develop a sculpture most nearly akin to that of Placopecten clintonius (Say) come from warmer waters. The pectens described by Say in 1824 are among the most common in the Miocene faunas of the middle States of the Atlantic seaboard. They were included in "a very large and fine collection of fossil shells which Mr. John Finch obtained with much labor and some expense in Maryland and which that gentleman with great liberality submitted" to Thomas Say for examination. The following footnote attends Pecten Clintonius: "Mr. Finch requested that three species of his collection that might prove to be new should be dedicated to the distinguished men whose names these shells bear." Thomas Jefferson, James Madison, and DeWitt Clinton were thus honored. DeWitt Clinton was in 1824 a prominent national figure. He had been United States Senator from New York, mayor of New York City, governor of New York, and in 1812 a formidable candidate for the presidency. The Great Lakes to Hudson River Canal was begun and completed largely through his efforts. He was an educator and a naturalist, the second president of the American Academy of Art, and the founder and an early president of the New York Historical Society. Mansfield (op. cit., p. 178) has expressed on the printed page the doubt held by many students that the Finch collection included only Maryland shells. It is highly probably that not only Pecten clintonius Say but also Say's P. jeffersonius and P. septenarius came from the Yorktown formation in Virginia, at a horizon higher than any recognized in the Miocene section of Maryland.» GARDNER, J. 1943. Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina. Part 1. Pelecypoda. United States Geological Survey Professional Paper, 199-A: 1-178, pls. 1-23. [p. 37, 38]
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Chlamys (Placopecten) clintonia (Say) Verrill; J. Gardner, 1943, Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina, plate 6, figures 1, 4.
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«The holotype, a left valve, is uniformly and rather finely threaded with rounded closly spaced, elevated, scabrorus threads, which extend across the sub-margins and auricles. About 31 mm. from the beak the radials become more scabrous, and intercostals appear. Auricles subequal.
Scabrous sculpture is well developed on valves from Murfreesboro Landing, Meherrin River, N. C. Shells from that locality are more equally, and less convex than those from other localities. Camptonectes marking somewhat less well developed than on valves from other localities. Young individuals from Bellefield, Va., have a rather poorly developed radial sculpture, and conspicuous camptonected marking. extending to the auricles. Wide byssal fasciole; ctenolium. Valves more ovate than those of adults. Auricles small, equal, radially sculptured; only slight flexure of byssal auricle in adults. Interior obsurelly grooved, or smooth. REMARKS. — C. magellanicus Gmelin resembles clintonius, and may be its descendant. It differs from clintonius in having a shorter hinge line, and a much narrower chondrophore. The radial threads of clintonius are somewhat coarser than those of magellanicus. In each species the ctenolium is present in the young individuals. DIMENSIONS. — Holotype, height 66, width 61 mm. Hypotype, height 111, width 105 mm. LOCALITIES. — Maryland (type), Say; Kingsmill, Bellefield, Evergreen, Coggins Point, Grove Wharf, Lanexa, Claremont Wharf, Yorktown, Va.; Murfreesboro Landing, Meherrin River, N. C. HORIZON. — St. Marys, and Yorktown (Miocene). HYPOTYPE. — Collection Cornell University. HOLOTYPE. — Academy of Natural Sciences of Philadelphia.» TUCKER-ROWLAND, H. I. 1938. The Atlantic and Gulf Coast Tertiary Pectinidae of The United States. Section III Systematic Descriptions. Mémoires du Musée Royal D'Historie Naturelle de Belgique [2me Série], 13: 1-76, pls. 1-6. [p. 52, 53]
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Chlamys (Placopecten) cintonius (Say); H. I. Tucker-Rowland, 1938, The Atlantic and Gulf Coast Tertiary Pectinidae of The United States, pl. 1, fig. 11.
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