Leptopecten pabloensis (Conrad, 1857)
CONRAD, T. A. 1857. Description of the Tertiary fossils collected on the survey [Williamson's survey]. In: Reports of explorations and surveys railroad from the Mississippi River to the Pacific Ocean. Senate Executive Document, 78 and House of Representatives Executive Document, 91, vol. 6: 69-73, pls. 2-5 (Reprinted in Dall 1909: 176-179) [p. 71 (Reprint Dall 1909, p. 178), pl. 3, fig. 14]
1857 Pecten pabloensis Conrad, 1857
«Comparison.—"This species appears to be a degenerate offshoot of P. discus, with which it occurs and with which it intergrades. The humped-up form may have been caused by the influx of more or less fresh water, and this suggestion is strengthened by the fact that pabloensis sometimes occurs very abundantly all by itself in beds two or three feet thick." (Grant and Gale, 1931, p. 199)
Pacipecten pabloensis has a thinner shell, the ribs become obsolete at the ventral margins, and the byssal notch on the right anterior ear is twice as wide as on P. discus. (Vedder and Moore, 1976, p. 121) Comments.— The right valve figured by Arnold (1906, pl. 27, figs. 6, 6a) is moderately inflated and has 19 ribs that are narrow and subrounded and prominent until the shell reaches a height of about 10 mm at which point they begin to broaden and become flat. At the ventral margin the ribs are broad and flat and demarcated by a deep, incised groove on both sides. Three barely perceptible interribs appear near the ventral anterior margin. The right anterior auricle has five slightly nodose riblets; the right posterior auricle has six faint riblets. The anterior dorsal margin is concave; the posterior is almost straight. The left valve figured by Arnold (1906, pl. 27, figs.5, 5a) is almost flat and has 23 very narrow ribs separated by interspaces up to four times as wide, which commonly have an interrib near the ventral margin. The left anterior auricle has seven riblets crossed by concentric lamellae which produce small nodes where bunched on the riblets. The left posterior auricle has six riblets sculptured as they are on the left anterior auricle. Pacipecten pabloensis resembles P. bilineatus (Clark) in sculpture but can be distinguished by the outline of the shell. ln Pacipecten bilineatus the shell flares outward at an angle slightly subparallel to the hinge on both the anterior and posterior sides; on P. pabloensis the posterior margin slopes evenly toward the ventral edge. Geographic range.— Middle to southern California. Geologic range.— Miocene. Occurrence in the Californias.— Briones Sandstone (Weaver, 1953), Cierbo Sandstone (Clark, 1915: Hall, 1960), Modelo Formation (Hudson and Craig, 1929), Neroly Sandstone (Hall, 1960), and San Pablo Formation (Hertlein, 1951).» 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. B48]
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Pacipecten pabloensis (Conrad) of Arnold; E. J. Moore, 1984, Tertiary Marine Pelecypods of California: Propeamussidae and Pectinidae, plate 15, figures 1, 3.
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«Description.— Shell averaging about 35 millimeters in altitude, about as high as long, equivalve, equilateral, thin, only slightly ventricose, and with smooth margins; sides sloping at a low angle and quite prominently concave above. Right valve with 18 to 20 square, flat-topped, radiating ribs, which are narrower and less distinct laterally and become more feeble as the periphery of the disk is approached; interspaces about as wide as the ribs, flat bottomed and often separated from the principal ribs by impressed lines which give the interspaces the appearance of bearing intercalary riblets; whole surface of disk sculptured by fine concentric lines and, in some instances, more or less prominent lines showing interrupted growth; hinge line somewhat longer than one-half length of disk; anterior ear slightly longer than posterior, deeply notched, and ornamented by 4 or 5 coarse radiating ridges and sharp, raised incremental lines; posterior ear slightly concave behind, and covered by numerous incremental lines and several obsolete radiating ribs. Left valve similar to the right except that in some cases the ribs are slightly more rounded above, and the looped or wavy concentric lines are more conspicuous; ears similar to those of the right .. valve except that the anterior one lacks the deep byssal notch. Hinge as in P. circularis except that the cardinal crura are very faint, while the auricular crura are quite prominent.
Dimensions.— Alt. 35 mm.; long. 35 mm.; hinge line 20 mm.; diameter 6 mm. The measurements given above are from an average-sized specimen. One left valve examined was 50 millimeters in altitude, but this size is exceptional for specimens from the type locality (shore of San Pablo Bay). The prominence of the ribs seems to be the most variable factor in this species. As a rule the ribs are sharp and well defined until the shell attains an altitude of 10 or 15 millimeters, when they broaden out and become low, those near the sides almost losing their identity in some specimens. One specimen examined retained the elevated and sharply defined ribs up to an altitude of over 25 millimeters, and very much resembled a young P. circularis var. aequisulcatus. P. pabloensis is so far known only in the San Pablo, or upper Miocene formation. At the type locality, on the shore of San Pablo Bay, Contra Costa County, a bed several feet thick is made up almost entirely of valves of this species. A.t this locality P. pabloensis is associated with P. crassicardo and P. andersoni ( ?). RANGE
Miocene (upper). Shores of San Pablo Bay, Contra Costa County (Newberry; Merriam; Arnold).»
ARNOLD, R. 1906. The Tertiary and Quaternary pectens of California. United States Geological Survey Professional Paper, 47: 1-264, pls. 1-53. [p. 88, 89]
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Pecten (Plagioctenium) pabloensis Conrad; R. Arnold, 1906, The Tertiary and Quaternary pectens of California, plate 27, figures 5, 5a, 6, 6a, 7.
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