Chlamys sespeensis (Arnold, 1906)
ARNOLD, R. 1906. The Tertiary and Quaternary pectens of California. United States Geological Survey Professional Paper, 47: 1-264, pls. 1-53. [p. 69, pl. 8, figs. 2, 2a, 3]
1906 Pecten (Chlamys) sespeensis Arnold, 1906
1906 Pecten (Chlamys) sespeensis var. hydei Arnold, 1906
1928 Pecten (Chlamys) erici Wiedey, 1928
1906 Pecten (Chlamys) sespeensis var. hydei Arnold, 1906
1928 Pecten (Chlamys) erici Wiedey, 1928
R. Arnold, 1906, plate 8.
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«1888. Pecten hastatus Cooper (not Sowerby), Seventh Ann. Rept. Cal. St. Min., 1888, p. 257 (Miocene of Estrella, San Luis Obispo County?).
Description.— Shell averaging about 50 millimeters in altitude, higher than long, inequivalve but both valves equally slightly convex, rather thin, and equilateral, except for ears: Right valve. with about 9 pairs of narrow, rounded, nearly smooth ribs, each pair being separated from the adjacent ones by interspaces, which are generally slightly wider than the interspace between the members of the pair; surface sculptured by incremental lines; hinge line about two-thirds length of disk; anterior ear much produced, sculptured by several prominent radials and numerous incremental lines; posterior ear over one-half as long as anterior, and sculptured similarly; byssal notch prominent. Left valve with about nine prominent, narrow, squarish, somewhat convex-topped ribs separated by much broader interspaces, along the bottom of which run one or more thread-like intercalaries of greater or less prominence; ears similar to those of the right valve.
Dimensions.— Alt. 37 mm.; long. 30 mm.; hinge line 19 mm.; diameter 12 mm.; umbonal angle 75°. This species is closely allied to, and is probably the precursor of P. hastatus Sowerby. It corresponds to P. hatatus in size, shape, and number of major ribs, but differs from the latter in having longer anterior ears and scarcely any secondary sculpture of minor riblets and scaly surface. P. sespeensis approaches more closely to the smooth form of P. hastatus found in the Pliocene, but may be easily distinguished from that form by its longer posterior ears and lack of much secondary sculpture. The only specimens of this species known are rather poorly preserved casts, with now and then small fragments of the shell hanging to them, which come from beds of lower Miocene age, locally known as "oil rocks," in the Sespe oil district, Ventura County. Type and cotype, No. 12480, California State Mining Bureau. Other specimens of the same species, Nos. 12427 and 14759 C. S. M. B. On, Tar Creek, Ventura County, Mr. Eldridge found P. sespeensis associated with Turritella (cf.) hoffmanni, Ostrea sp., and Modiolus sp. RANGE.
Miocene (lower.) Sespe oil district, Ventura County (Watts); Tar Creek, Ventura County (Eldridge).»
RALPH ARNOLD, 1906
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«The internal molds serving as types of Pecten yneziana subyneziana are from the undivided Sacate and Gaviota Formations (UCMP B-6963) or "Coldwater" sandstone of Weaver and Kleinpell (UCMP B-6940). These and other small Paleogene Chlamys, such as those referred by authors to Chlamys sespeensis (Arnold), are possibly ancestral to V. yneziana but they were not considered in this study.»
SMITH, J. T. 1991. Cenozoic Giant Pectinids from California and the Tertiary Caribbean Province: Lyropecten, "Macrochlamis", Vertipecten, and Nodipecten species. United States Geological Survey Professional Paper, 1391: v + 1-155, figs. 1-18, pls. 1-38. [p. 86]
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«Comments.— On the basis of a specimen figured by Weaver and Kleinpell (1963, pl. 29, fig. 10; pl. 30, fig. 1) MacNeil (1967, p. 38) considered Chlamys sespeensis to be "very close to the boundari between Chlamys and Vertipecten, morphologically." The hypotypes of Loel and Corey (1932) are typical Chlamys, and sespeensis is here retained in that genus. A few of the left-valve ribs are more prominent than others, but this is true for other Chlamys. The valves are equilateral, and the figured right valve has about 25 low rounded ribs with equal interspaces, some bearing an interrib. The right
anterior auricle has six fine riblets and a moderately large byssal sinus. The smaller right posterior auricle is smooth. The left valve figured has 27 rounded to subrounded ribs, some higher than others, separated by slightly wider interspaces bearing 1 or 2 interribs. The left-valve auricles seem to have been smooth. The valves of Chlamys hodgei are more inequilateral and the left valve much more finely sculptured than C. sespeensis. Chlamys branneri has almost square ribs on the left valve and strongly sculptured left anterior auricle, which differentiate it from C. sespeensis. Geographic range.— Oregon (Durham and others, 1942); Southern California. Geologic range.— Eocene to Miocene. Occurrence in California.— Eocene: Coldwater Sandstone (Weaver and Kleinpell, 1963); Oligocene: upper part of Gaviota Formation (Weaver and Kleinpell, 1963); Oligocene and Miocene: Rincon Shale (Avila and Weaver, 1969) and Vaqueros Formation (Kew, 1924; Loel and Corey, 1932; Dibblee, 1950; Squires and Fritsche, 1978).» MOORE, E. J. 1984. Tertiary Marine Pelecypods of California: Propeamussidae and Pectinidae. United States Geological Survey Professional Paper, 1228-B: iv + B1–B112, figs. 1-2, pls. 1-42. [p. B20]
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Chlamys sespeensis (Arnold) of Loel and Corey; E. J. Moore, 1984, Tertiary Marine Pelecypods of California, plate 4, figures 2, 3.
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«Pecten (Chlamys) sespeensis Arnold is one of those most important Vaqueros species which are very abundant, have wide distribution geographically and stratigraphically within the Vaqueros horizon, and are distinctive and easily recognizable forms.
Collections from many localities from Orange County to Monterey County show decisively that the species P. sespeensis Arnold is a most variable Pecten, and that all variants are properly included under the original name Pecten (Chlamys) sespeensis Arnold. The type of P. sespeensis Arnold, from "Sespe Creek Oil Fields" is admittedly of very poor preservation. The writers in collecting these, found that the beds in which the Vaqueros fossils occur are considerably hardened and fractured, so that the matrix does but rarely allow the obtaining of an exterior surface of a Pecten. Almost every specimen breaks out of the hard matrix with an inner shelly layer remaining on the cast within. These are smooth, devoid of secondary ribs or riblets and scaly surface, and are identical with the specimens figured by Arnold as P. sespeensis. But when the true outer surface of the shell is seen in softer matrix, or occasionally remaining on a cast, the sculpturing is identical with that of P. sespeensis var. hydei Arnold. The apparent pairing of ribs on the cast (less so on exterior) is due to alternation in the number of interculary ribs. In many other localities the inner layers show the same characters. Individual variation in P. sespeensis is remarkable. In any locality the number of strong ribs may vary from 18 to 29. The ribs' development is most variable in number, spacing, and strength of interculary ribs. The valve (which is usually thin) varies considerably in outline and is frequently constricted near periphery. Pecten branneri Arnold (1906) is in all probability equivalent to P. (Chlamys) sespeensis Arnold. Only poorly preserved casts were figured and described and these are identical with P. sespeensis in all discernible characters. Forms found north of Lynch's Mountain, Monterey County, have usually been referred to P. branneri, while in that vicinity P. sespeensis is commonly reported. P. sespeensis has been collected from localities at which "P. branneri" has been reported. Pecten (ChIamys) erici Wiedey (loc. cit.) is one of the common variants in which the main interculary or secondary ribs are developed to equal strength with the primary 18 ribs all of which, counted with the lateral ribs, total about 30. Pecten (Chlamys) sespeensis is usually found associated with Anomia vaquerosensis, n. sp., Turitella inezana Conrad (or varieties), and Spondylus perrini Wiedey or Pecten magnolia. Geologic range.— Vaqueros horizon, Lower Miocene only. Distribution.— Rare in Orange County, west of Point Dume, western Santa Monica Mountains, Los Angeles County; very abundant in the Ventura Basin region at Oak Ridge and South Mountain, Sespe Creeks, Ventura River, and Ojai Valley; common on the Channel Islands, in western Santa Ynez Mountains, Santa Barbara County; found in San Emigdio region, and Kern River district, Kern County; and in the La Panza Mountains, eastern San Luis Obispo County; common in the Bryson quadrangle region and Junipero Serra quadrangle of Monterey County, and in the Santa Cruz quadrangle (= P. branneri). Also seen in well cores from the south end of the San Joaquin Valley in association with Turritella inezana Conrad (exact horizon and locality not revealed).» LOEL, W. & W. H. COREY. 1932. The Vaqueros formation, lower Miocene of California I. Paleontology. University of California Publications, Bulletin of the Department of Geological Sciences, 22 (3): 31-410, pls. 4-65. [p. 197]
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Pecten (Chlamys) sespeensis Arnold; W. Loel & W. H: Corey, 1932, The Vaqueros formation, lower Miocene of California I. Paleontology, plate 21, figures 1-5.
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