Pecten (Lyropecten) gabbi Clark, 1918
CLARK, B. L. 1918. The San Lorenzo Series of middle California. A stratigraphic and palaeontologic study of the San Lorenzo Oligocene series of the general region of Mount Diablo, California. University of California Publications, Bulletin of the Department of Geological Sciences, 11 (2): 45-234, pls. 3-24, 4 text figs. [p. 131, pl. 15, figs. 1-2]
«PECTEN (LYROPECTEN) GABBI, n. sp.
Plate 15, figures 1 and 2 Type specimen 11138, Coll. Invert. Palae. Univ. Calif., loc. 1311 Shell fairly large, heavy, equivalved; right valve flat; left valve convex. Apical angle about 100º; dorsal edges very nearly straight; ventral edge rather strongly convex. Left valve sculptured by about nineteen prominent and variable, rounded radiating ribs; on the type specimen, a left valve, seven of these ribs are heavier than the others; one radiating from the beak down to the middle of the ventral edge, three of the other ribs being anterior and three posterior to this; in each interspace between these major ribs is a less prominent and narrower rib, and in the interspaces between this narrower rib and the major ribs on each side of it is a still less prominent and more narrow rib; besides this, in some of the interspaces, there is a fourth set of ribs still less prominent than the others. The valve originally had strong and beautiful imbricated sculpturing, which does not show on the type specimen but is seen on some of the external easts. Ears large and heavy. One specimen of a cast of the anterior ear shows eight or nine radiating ribs; the ones next the dorsal margin beingmuch finer than the others; surface of ear strongly imbricated. Eight valve flat, sculptured by nineteen or twenty fairly heavy, rather broadly rounded, radiating, somewhat variable ribs; as on the left valve there is a tendency for some of the ribs to be heavier than the others, but this difference is not so great nor is the distribution of the heavy ribs so regular ; apparently there were no riblets in the interspaces. Surface of right valve sculptured by imbricated incremental lines, which are coarser than on the left valve. Sculpturing of ears of right valve not preserved.
Dimensions.— Type specimen: height, about 55 mm.; length, about 46 mm.; diameter of both valves, about 37 mm. Occurrence.— University of California locality 1311. This form appears to be related to Pecten perrini Arnold, [105] which is very common in the Vaqueros, the Turritella inezana zone, of the southern part of the state. P. perrini is a very variable species, the sculpturing being more irregular than on P. gabbi; it has about the same number of ribs, and on some specimens certain of the ribs stand out more prominently than others, as is the case with P. gabbi. The imbricated concentric sculpturing appears to be much stronger on the former, is more irregular, and the inter-ribbing and striations are heavier. P. fucanus Dall [106] is of the same general type as P. gabbi; it differs from this form in that it has a less number of ribs, the interspaces are wider, and the radial striations, seen on the ribs of the former, are lacking on the latter.» [105] For original description of P. perrini, see U. S. Geol. Surv., Prof, paper no. 47, p. 80, pl. 14, figs. 1, 1a; pl. 15, fig. 1, 1906.
[106] For original description of P. fucanus see Trans. Wagner Free Inst. Sci., vol. 3, pt. 4, p. 704, pl. 26, fig. 7, 1898. For good redescription, see Arnold, U. S. Geol. Surv., Prof. paper no. 47, p. 66, pl. 10, figs. 1, 2 and 2a, 1906. BRUCE LAWRENCE CLARK, 1918
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B. L. Clark, 1918, plate 15.
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