Amussiopecten lompocensis (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. 92, pl. 28, figs. 1-3]
1906 Pecten (Amusium) lompocensis Arnold, 1906
R. Arnold, 1906, plate 28.
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«Description. — Adult shell averaging about 100 millimeters in altitude, somewhat longer than high, compressed, subequivalve, equilateral, and thin; base regularly rounded; sides straight, or nearly so; margins smooth. Right valve with external surface smooth except for very tine and numerous concentric and nearly obsolete radiating striae; internal surface sculptured by about fourteen pairs of rather prominent rounded radiating lirae, the members of each pair being separated by flat spaces usually of about two-thirds the width of the major interspaces (in one case the spaces between the members of each pair were as wide as the interspaces); hinge line about two-fifths the length of the disk; ears subequal, obliquely truncated, and finely concentrically sculptured; anterior with a faint suggestion of a byssal notch; ctenolium absent. Left valve similar in all respects to the right except that the anterior ear has a straight, rather than an arcuate end.
Dimensions. — Alt. 103 mm.; long. 113 mm.; hinge line 40 mm.; diameter 20 mm. This very unique species, of which there are no closely allied forms in the Tertiary or living faunas of the west coast, so far as known, is somewhat analogous to P. (Amusium) mortoni Ravenel of the Miocene and Pliocene of eastern America. It is distinguishable from the latter species, however, by the smaller number, greater prominence, and more regular arrangement of the internal ribs. P. lompocensis is included in Amusium, because it appears to show stronger affinities for this than for any other group. P. lompocensis was first discovered in the limy beds which outcrop about 4 miles south of Lompoc, Santa Barbara County, where the types, which are now in the collection of the California Academy of Sciences, were collected by Mr. Frank M. Anderson. At the type locality this unique form is found associated with the following species in beds of probable lower Miocene age: Pecten crassicardo, Ostrea (aff.) titan, Lima n. sp., Cidaris (?) sp., Terebratella 2 sp., and Scutella sp. On the ridge between the Upper and Lower Ojai Valleys, Ventura Cuuuty. Mr. Eldridge has found P. lompocensis with the following species, in beds which he thinks are of lower Miocene (Vaqueros) age: Pecten crassicardo, Turritella (cf.) hofffmanni, Phacoides sp. (flat), Ostrea sp., Nassa sp., Dentalium sp., Chione sp., Leda sp. Balanus sp., Tritonium or Fusus sp. RANGE
Miocene (lower ?). Four miles south of Lompoc, Santa Barbara County (F. M. Anderson); ridge between upper and lower Ojai valleys, Ventura County (Eldridge).»
RALPH ARNOLD, 1906
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«Comments.— The holotype of A. Iompocensis was broken after it was originally illustrated, perhaps as a result of the San Francisco fire in 1906, and now consists of the posterior half of the left valve. No shell remains on the specimen but the paired internal lirae are clearly visible and are more closely spaced near the posterior margin.
Geographic range.— Middle to southern California. Geologic range.— Miocene. Occurrence in the California.—Miocene: Branch Canyon Sandstone (Hill and others, 1958; Vedder, 19?3), Monterey Shale (Kleinpell and Weaver, 1963), Obispo Formation (Hall and others, 1966), San Onofre Breccia (Addicott, 1974), Temblor "Horizon" (Loel and Corey, 1932), and Tierra Redonda (Durham, 1970) Formations, Tranquillon Volcanics (Dibblee, 1950); unnamed Miocene strata north of Bald Mountain in Santa Barbara County (Vedder and others, 1967), and unnamed Miocene strata, San Clemente Island (Susuki and Stadum, 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. B67]
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Amusium lompocensis (Arnold); E. J. Moore, 1984, Tertiary Marine Pelecypods of California: Propeamussidae and Pectinidae, plate 2, figures 10, 11.
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«PECTEN (AMUSSIOPECTEN) LOMPOCENSIS Arnold is common in the basal part of the orange sandstone exposed at section RS-3, loc. 6317. Many of the specimens are found articulated, but complete specimens are difficult to collect. Each shell is fractured and it is necessary to coat the surface with an adhesive, prior to removing it from the coarse sandstone matrix.
P. (A.) lompocensis is often confused with Pecten (Amussiopecten) vanvlecki Arnold, (Arnold 1907) and the two species are not readily separable unless the external and internal surfaces are well preserved. However, there are several helpful distinguishing features, which include the development of the external radial ribs and the spacing of the internal ribs. On the right valve of P. (A.) lompocensis, the external radial ribs are poorly defined and the ribs are paired internally. On the left valve, the external ribs are better developed and the internal ribs are not paired but equally spaced with the interspaces. In P. (A.) vanvlecki, the external ribs on both valves are betler developed but internally, the paired ribs occur on the left valve and they are equally spaced in the right valve. Masuda (1971), in his study of Amussiopecten, stated that P. (A.) vanvlecki is easily distinguishable from the frequently confused species P. (A.) lompocensis by "its rather thick shell, squarish, low, flatly round-topped radial ribs lending to become obsolete towards the ventral and lateral margins, paired internal ribs developed at lower part and larger and angulate auricles." Masuda (1971) did not make any distinction as to which valve is characterized by the paired internal ribs. The occurrence of fragmentary specimens of Amussiopecten cf. A. vanvlecki is reported by Vedder and Moore (1976) from China Canyon in the southern end of San Clemente Island in association with Lyropecten crassicardo (Conrad) and Crassostrea cf. C. freudenbergi (Hertlein and Jordan), USGS loco M6505. According to Vedder and Moore (1976), these fossils are imbedded in a fine-to coarse-grained calcarenite which differs considerably in lithology from the orange sandstone in which P. (A.) lompocensis occurs so commonly (UCLA loc. 6317, which apparently has not been visited by previous investigators).» SUSUKI, T. & C. J. STADUM. 1978. A Neogene section, northeastern San Clemente Island, California. Contributions in Science, 299: 1-24, figs. 1-29 [p. 11, 12]
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Pecten (Amussiopecten) lompocensis Arnold; T. Susuki & C. J. Stadum, 1978, A Neogene section, northeastern San Clemente Island, California, figures 12-17.
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