Leptopecten bilineatus (Clark, 1915)
CLARK, B. L. 1915. Fauna of the San Pablo Group of middle California. University of California Publications Department of Geology Bulletin, 8 (22): 385-572, pls. 42-71. [p. 448, pl. 64, figs. 3, 4]
1915 Pecten (Plagioctenium) bilineatus Clark, 1915
B. L. Clark, 1915, plate 64.
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«Shell thin, medium in size, equivalved and equilateral, usually longer than high. Dorsal edges gently concave, the anterior dorsal edge usually being shorter than the posterior; ventral edge quite strongly and regularly arcuate. Apical angle averaging about 102º. Hinge plate about half as long as the length of the shell. Both valves gently convex and similarly sculptured by 18 to 20 low, flat-topped, radiating ribs with interspaces little wider than the tops of the ribs. In each interspace and almost filling it there is a low, broad, flattopped interrib. On some specimens these broad interribs become almost as prominent as the ribs themselves; this is more especially noticeable near the dorsal edges, where the ribbing becomes finer. Anterior ear of right valve notched, bearing three to four fine, radiating riblets; posterior edge of posterior ear slightly oblique, bearing very faint, almost obsolete, radiating riblets. Ears of left valve similar to those of the right valve, except that the anterior ear is not notched.
P. bilineatus, n. sp., has been confused with P. raymondi, n. sp., and P. pabloensis Conrad, which species also occur in the same locality. Arnold's description of P. pabloensis Conrad (U. S. G. S. Prof. Paper, No. 47, pp. 87-89) in part applies very well to P. bilineatus rather than to what is taken by the writer as being the true P. pabloensis Conrad. Arnold figured P. pabloensis as three distinct species, which are seen on Plate XXVII of the publication mentioned above. Figure 5 represents a very good specimen of the left valve of P. pabloensis Conrad. Figure 6 represents a good specimen of a right valve of P. bilineatus, n. sp., while Fig. 7 appears to represent a left valve of P. raymondi, n. sp. P. bilineatus, n. sp., differs from P. pabloensis Conrad in that the ribs are wider and not so far apart, they do not show the tendency to split as in the other species, the interrib is much broader. The concentric sculpturing is difl'erent, on P. pabloensis Conrad it is distinctly imbricated, the species belonging to the same subgenus as Pecten hastatus Sowerby — that is, Chlamys; on P. bilineatus it is not imbricated. P. bilineatus resembles P. raymondi, n. sp., in the number and width of the ribs, in the width of the interspaces, and in general outline; it differs from that species in the following respects: it has a thinner shell, the ribs are not so prominent; on P. raymondi there is no interrib; the two valves of the latter are unequal, the right valve being flat and the left valve convex, thus belonging to the subgenus Pecten; the valves of P. bilineatus are both convex, the species belonging to the subgenus Plagioctenium. Dimensions.— Of type (pl. 64, fig. 3), a right valve, length 45 mm.; height 38 mm. Occurrence.— This species occurs in the famous Pecten beds on San Pablo Bay near the town of Kodeo, University of California locality 1632.» BRUCE LAWRENCE CLARK, 1915
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