1915] 



Clark: Fauna of the San Pablo Group 



451 



radiating ribs about equal in width to that of ribs. Concentric lines 

 of growth usually 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 IT. 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 imbri- 

 cated 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 dichotomously, 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 Avell 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 Dall'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, mis- 

 taken for P. andersoni Arnold. Both species belong to the subgenus 

 Pecten, having the left valve convex and the right valve fiat. P. ray- 

 mondi, n. sp., differs from P. andersoni Arnold in the following re- 

 spects: 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 



