Syncyclonema halli (Gab, 1861)
GABB, W. M. 1861. Synopsis of the Mollusca of the Cretaceous Formation, Including the Geographical and Stratigraphical Range and Synonymy. Proceedings of the American Philosophical Society, 8 (65): 57-257. [p. 214]
1856 Pecten rigida Hall & Meek, 1856
1861 Pecten hallii Gabb, 1861 [nomen novum pro Pecten rigida Hall & Meek, 1856; non Pecten rigida J. Sowerby, 1818]
1861 Pecten hallii Gabb, 1861 [nomen novum pro Pecten rigida Hall & Meek, 1856; non Pecten rigida J. Sowerby, 1818]
«Syncyclonema halli (Gabb)
(PI. 1, figs. 1-9; pl. 2, figs. 1-5; pl. 3, figs. 1, 2, 4-6)
Pecten rigida Hall & Meek, 1856, p. 381, pl. 1, figs. 4a-c (non Sowerby, 1818, p. 5, pl. 205, fig. 8).
Pecten hallii Gabb, 1891, p. 214. (Nom. nov. for P. rigida Hall & Meek). Sincyclonema rigida (Hall & Meek). Meek, 1864a, p. 7. Syncyclonema rigida (Hall & Meek). Meek, 1864a, p. 31. Meek, 1876, p. 27, pl. 16, figs. 5a, b. Whitfield, 1880, p. 383, pl. 7, fig. 1. LECTOTYPE. Here selected (p. 7), AMNH 9347/1.1, a right valve, 4 mm long and 5 mm high (pl. 1, fig. 3), Sage Creek, South Dakota, Pierre Shale, Upper Campanian-Maastrichtian.
MATERIAL. One hundred and seventeen specimens, including eight articulated or displaced bivalved.
OCCURRENCE. In the type area of the Fox Hills Formation S. halli is virtually restricted to the Timber Lake Member (Waage, in press).
The species, recorded by Fisher, et al. (1960), as Syncyclonema hallii, from the lower part of the Pierre Shale and in the Sego Sandstone in western Colorado and eastern Utah, and by Griffitts (1949) as S. rigida, from the "Rocky Ridge sandstone member of the Hygiene zone" of the Pierre Formation in Colorado, is apparently rare in the Upper Cretaceous sequences of the Western interior. It is found mainly in sandstone units. DESCRIPTION. Small, length of specimens 3.5 to 13.2 mm, subequivalve, equilateral to slightly inequilateral, compressed, valves equally inflated. Orbicular, with height, width of one valve, length of dorsal margin (between extremities of ears), and anterior length respectively, 101.4 to 129.2 (n=28, x̄=111.7%), 4.6 to 13.3 (n=25, x̄=9.1%), 48.2 to 64.6 (n=19, x̄=54.9%), and 45 to 55.9 (n=28, x̄=49.2%) per cent of length, and length of anterior ear 50 to 63.2 (n=19, x̄=55.7%) per cent of length of dorsal margin. Umbones weakly prosogyrous, project slightly above dorsal margin. Antero- and postero-dorsal margins of the disc of shell straight or slightly concave, form an angle of 87.5 to 100 degrees, and meet the rounded ventral margin at distinct angulations. Dorsal margins of ears straight, intersect at a small angle, the anterior is more steeply inclined. Margin of right valve, especially that of the anterior ear, projects above and overlaps the margin of left valve. Anterior ear equal to or larger than posterior. Ears of left valve, and posterior ear of right valve, weakly delimited from disc of shell, although the margin of disc becomes overhanging ventrally, with straight or rounded anterior or posterior margins which form a rounded or angled junction with the dorsal margin of the ear. Anterior ear of right valve sharply delimited from the overhanging margin of the disc, with a narrow fascicle at base, at the ventral end of which is a narrow moderately deep V-shaped byssal sinus. Ctenolium apparently lacking.
Ornament similar on both valves, of submicroscopic vermiculating Camptonectes-striae, 40 to 50 per millimeter at ventral margin, fine growth striae and coarser irregular growth lamellae tending to give a reticulate pattern. Striae are sometimes very weak on the center of disc and, with the growth striae, are often very strong on the antero- and postero-dorsal flanks of disc and sometimes on ears. Either the striae or growth lamellae may dominate on the ears. Internal surface of shell sometimes with irregular or regular shallow rounded concentric costae and fine radial striae. Resilifers triangular, internal, do not extend to ventral margin of hinge-plate. Resilifer of right valve bordered on either side by a short strong tooth-like process which extends the length of the resilifer and fits into sockets on each side of the resilifer of the left valve. Ligament groove narrow, submarginal, extends most of length of ears, bordered below by a weak cardinal crura. No auricular crura. Pallial line continuous, extending in a loop across shell at about one-quarter of the height above base, becomes indefinite antero-dorsally at base of the fasciole. The pallial line has a small subquadrangular scar situated at about one-third the height of the shell; and a large subcircular adductor scar is situated near the base of the posterior ear in the dorsal quarter of the shell. Muscle scars on the left valve unknown. Ostracum less than 0.2 mm thick, of three layers. A very thin outer layer, probably prismatic, bearing the Camptonectes striae, a translucent middle layer, apparently of zigzag lamellar structure (Bøggild, 1930, p. 267), that thickens towards the ventral margin, and an inner white layer of complex cross-lamellar structure that is thickest in the umbonal region and ceases within one to two millimeters of ventral margin (pl. 2, fig. 1). Outer surface vitreous, but with irregularly distributed, radially elongated patches of matt appearance (pl. 2, fig. 4). DISCUSSION. Left and right valves of S. halli are equally inflated and their other measurements have approximately the same range of values and means. In spite of the extremely fragile shell few specimens from concretions in the type area were broken during deposition. Complete specimens are very difficult to extract and the ears are especially prone to damage. At Yale loc. 288, Solen, North Dakota, where the sediment is a limonite-cemented medium-grained sandstone with little argillaceous matrix, many specimens were broken during transportation. The tripartite layering of the shell described above requires confirmation by thin section studies. As interpreted it is similar to the shell of "P. textorius (Liassic)" of Bøggild (1930, p. 267). The very thin outer prismatic layer bears the Camptonectes-like striae and, although always present on well preserved specimens, it is easily removed by abrasion, weathering or extraction. The shell then appears smooth or with concentric lamellae only. Species of Camptonectes recorded in the literature as being smooth and striate should be re-examined to ascertain that the outer layer, if this is characteristic of the genus, has not been lost, especially in the case of small species.
Whitfield (1880, pl. 7, fig. 1) illustrated a different pattern of muscle scars on a steinkern from "the forks of the Cheyenne River, Black Hills." He commented on the unusual position of the muscle scar. Examination of recent pectinid species held in the collections of the Peabody Museum, Yale University, showed that some, for example Cyclopecten (Delectopecten) vitreus (Chemnitz), Pseudamussium striatum (Müller), and P. septemradiatum (Müller), have an elongated or oval scar in a comparable position close to the base of the posterior ear of the right valve. The ventral small subquadrangular scar in the pallial line is present on several specimens and seems to be a valid scar. It may represent a gill suspensory attachment (Newell, 1937, p. 21). COMPARISONS. Specimens of Pecten (Camptonectes) kaufmanensis Stephenson (1941, pl. 21, figs. 7-9) and P. (C.) cavanus Stephenson (1952, pl. 19, fig. 7) closely resemble S. halli. Other specimens from Gulf and Atlantic Coastal Plains sequences classed under Pecten or Syncyclonema resemble S. halli in shape and size but apparently lack the fine Camptonectes-striae. Some, for example P. (Syncyclonema) travisanus Stephenson (1941, pl. 22, fig. 1), show traces of radial ornament, while others, like P. simplicius Conrad (Wade, 1926, pl. 20, fig. 7) and the right valve of P. travisanus (Stephenson, 1941, pl. 22, fig. 2), have relatively strong lamellae like S. halli when its outer prismatic layer is lost.»
SPEDEN, I. G. 1967. Revision of Syncyclonema (Upper Cretaceous) and comparison with other small pectinid bivalves and Entolium. Postilla, 110: 1-36, pls. 1-7. [p. 10-13]
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Plate 1.
Plate 2.
Plate 3.
Syncyclonema halli (Gabb); I. G. Speden, 1967, Revision of Syncyclonema (Upper Cretaceous) and comparison with other small pectinid bivalves and Entolium, plate 1, figures 1-9; plate 2, figures 1-5; plate 3, figures 1, 2, 4-6.
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