Camptonectes hilgardi (Stephenson, 1923)
STEPHENSON, L. W. 1923. The Cretaceous formations of North Carolina, part 1. Invertebrate fossils of the Upper Cretaceous formations. North Carolina Geological Survey, 5: 1-402, pls. 1-100. [p. 206, pl. 57, figs. 6, 7]
1923 Pecten hilgardi Stephenson, 1923
L. W. Stephenson, 1923, plate 57.
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«Description.— This species is represented in the collections by two specimens only, one from North Carolina and the other from Mississippi. The former is a fragment on which the surface is well preserved, and the latter is a much more complete specimen with the sculpture somewhat less perfectly preserved. The Mississippi specimen, a right valve, is designated the type.
Shell thin, very brittle, subovate in outline, subequilateral, depressed convex; beak small, depressed, not projecting, probably situated slightly back of the middle of the hinge line. Ears sharply delimited from the body of the shell, the delimiting lines diverging from the beak at an angle of 90 degrees. Anterior ear of right valve partly separated from the body of the shell by a profound byssal sinus; a shallow, rather wide sulcus extends from the inner end of the byssal sinus to the beak; posterior ear poorly preserved. Hinge and internal characters not uncovered. Approximate dimensions of the Mississippi specimen: Length 26 mm., height 25 mm., convexity 2 mm. The fragment from North Carolina belongs to a somewhat larger individual. Surface marked by numerous very fine, closely spaced, radiating ribs with narrower interspaces, and by prominent, thin, concentric lamellae spaced 1 to 3 mm. apart, which curl downward, touching the surface of the shell below and forming hollow, round-topped ridges. The radiating ribs are nearly as distinctly developed on these ridges as they are on the main surface of the shell. The concentric ridges become obscure toward the beak. The radiating ribs are reflected on the inner surface of the shell. The ears are crossed by the radiating ribs and by transverse ribs which, though smaller, correspond to the concentric ridges on the main body of the shell. Remarks.-- Pecten hilgardi is closely allied to P. perlamellosa Whitfield from Haddonfield, N. J., the types of which, two left valves in the collection of the Academy of Natural Sciences of Philadelphia, are more ventricose than the type of P. hilgardi, a right valve, though the latter may have been flattened by compression. The concentric lamellae of P. perlamellosa are more closely crowded and appear to stand up more prominently, not curling down and touching the surface below as in P. hilgardi. In view of these differences and the fact that the New Jersey specimens occur in beds stratigraphically lower than the deposits in which the North Carolina and Mississippi specimens are found, it seems best to regard the two forms as specifically distinct, at least until better material is available for study.
P. hilgardi is also closely related to P. argillensis Conrad, but differs in the greater prominence and more continuous development of the concentric lamellae. On P. argillensis these lamellae are as a rule largely destroyed, though their former presence is indicated by occasional remnants on the surface of the shell. Type.— Collection of the U. S. National Museum, cat. No. 31669. From near Troy, Pontotoc County, Miss. (6471). Named in honor of the late Prof. Eugene W. Hilgard, former State Geologist of Mississippi.
Occurrence in Mississippi.— Ripley formation (Exogyra costata zone): At an exposure on Northeast Cape Fear River, 58¾ miles above Wilmington (4149).
Occurrence in Mississippi.— Ripley formation (Exogyra costata zone): Public road, one-quarter of a mile east of Troy, Pontotoc County (6471).
Range.-- Exogyra costata zone.»
LLOYD WILLIAM STEPHENSON, 1923
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This species is questionably present in the collection from one locality in the Owl Creek formation of southeastern Missouri (USGS 16430). The material includes the external and internal molds of one individual and fragments of two external molds, all representing young stages of growth.
In form and radial ornamentation the species resembles that of Pecten (Camptonectes) bubonis Stephenson. The concentric scultpure is, however, much coarser; it consists of relatively prominent lamellae spaced in this young stage 1 millimeter or less apart; these lamellae curl downward touching the surface below, thus forming hollow round-crested ridges. In the holotype from Mississippi, a much larger shell, the spacing of the concentric lamellae reaches a maximum of 3 millimeters near the outer margin. A detailed description of the species is given by Stephenson (1923, p. 206). STEPHENSON, L. W. 1955. Owl Creek (Upper Cretaceous) fossils from Crowleys Ridge Southeastern Missouri. United States Geological Survey Professional Paper, 274-E: 97-140, pls. 14-24. [p. 113]
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Pecten (Camptonectes) hilgardi Stephenson?; L. W. Stephenson, 1955, Owl Creek (Upper Cretaceous) fossils from Crowleys Ridge Southeastern Missouri, plate 17, figure 5.
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