Pecten (Pecten) raymondi Clark, 1915
CLARK, B. L. 1915. Fauna of the San Pablo Group of middle California. University of California Publications, Bulletin of the Department of Geological Sciences, 8 (22): 385-572, pls. 42-71. [p. 450, pl. 46, figs. 1-2; pl. 47, figs. 1-2]
«PECTEN (PECTEN) RAYMONDI, n. sp.
Plate 46, figures 1 and 2; plate 47, figures 1 and 2
Shell rather heavy, subcircular, nearly equilateral, inequivalved; hinge line averaging slightly more than one half the length of the shell; height about equal to length. Anterior ears slightly longer than posterior ears. Apical angle about 102°. Dorsal edges slightly concave; posterior edge longer than anterior edge. Left valve ventricose, sculptured by seventeen to twenty squarish to slightly rounding, heavy, radiating ribs, the interspaces averaging about the same width as the ribs; on some specimens the ribbing tends to become a little finer toward the dorsal edges than on the middle of the disk. Surface also covered by medium fine to coarse concentric lines of growth. Anterior ear sculptured by three to five medium fine radiating ribs; anterior margin of ear slightly notched. Posterior ear with four to five radiating ribs, posterior edge straight, and sloping forwards. Right valve flat, sculptured by seventeen to twenty radiating flattopped ribs which are usually not as prominent as the ribs of the left valve. On some of the smaller specimens the right valve is slightly ventricose but never so much as the left valve. Interspaces between radiating ribs about equal in width to that of ribs. Concentric lines of growth usuall.y not so heavy as on left valve. Anterior ear deeply notched, sculptured by three to five medium fine radiating ribs.
This species has been confused with P. discus Conrad. Arnold in his paper "The Tertiary and Quaternary Pectens of California" (Prof. Paper U. S. G. S., no. 47) plate XXVII, fig. 1, figures under the name of P. discus a very good specimen of P. raymondi, n. sp. This came from the San Pablo near the town of Walnut Creek. Figure 2 on the same plate, also listed as P. discus Conrad, shows a rather broken specimen of a species which is common in the Monterey shale in the southern part of the state. This form is referred to by Dall (Trans. Wagner Free Inst. Sci., vol. 3. pts. 3-4, pp. 704. 705) as P. discus Conrad. The writer has some very good specimens of this last form from the Monterey shale. It is very different from P. raymondi, being equivalved, thinner and shows a distinct fine imbricated concentric sculpturing. It is somewhat similar to P. pabloensis Conrad in that sometimes a narrow riblet is seen in the interspace of the radiating ribs and there is a tendency for the major radiating ribs to divide diehotomously, also they become finer towards the dorsal margins of the shell. Neither P. raymondi, n. sp., nor the Monterey species referred to above, agrees with the original description or picture of P. discus Conrad. The type of P. discus Conrad evidently came from the Lower Miocene, Vaqueros or Monterey. Conrad's meagre description and his picture agree fairly well with P. andersoni Arnold, a characteristic Monterey or Temblor form. It therefore seems very probable that P. andersoni Arnold should be considered as a synonym of P. discus Conrad. Before the change is made, however, good material should be obtained from the section from which the type of P. discus Conrad was obtained. Therefore the writer for the present accepts Ball's determination of P. discus, leaving for the present P. andersoni as a legitimate species. P. raymondi, n. sp., may easily be, and has many times been, mistaken for P. andersoni Arnold. Both species belong to the subgenus Pecten, having the left valve convex and the right valve flat. P. raymondi, n. sp., differs from P. andersoni Arnold in the following respects: It has a larger number of radiating ribs; the interspaces are not so wide. On P. andersoni Arnold the radiating ribs of the left valve are much narrower at the top than at the bottom, the top of the rib tending to be subangulate. On P. raymondi, n. sp., the radiating ribs of the left valve are broadly rounded to flat-topped. The radiating ribs of the flat right valve of P. andersoni Arnold are distinctly rounded, but not as narrow-topped as those of the left valve. The radiating ribs of the right valve of P. raymondi are broad and usually flat-topped. P. andersoni Arnold appears to be characteristic of the Monterey or Temblor Horizon. The writer has never seen it in the Upper Miocene. P. raymondi, n. sp., is a common and a very characteristic species of the Upper Miocene. It is very common in the San Pablo of Middle California and also in the Scutella breweriana zone. It is also very common in the "Santa Margarita" in the southern part of the state. Named in honor of Professor W. J. Raymond of the University of California. Dimensions.— Large specimen figured, height, 50 mm.; length, 50 mm. ; diameter, 7 mm. Occurrence.— University of California localities 1942, 1947. 1946, 1478, 136, etc. ; also from the Seutella breweriana zone in Middle California and from the Santa Margarita of the southern part of the state.» BRUCE LAWRENCE CLARK, 1915
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B. L. Clark, 1915, plates 46, 47.
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