1915] 



Clark: Fauna of the San Pablo Group 



449 



rad. 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 con- 

 centric sculpturing is different, on P. pabloensis Conrad it is distinctly 

 imbricated, the species belonging to the same subgenus as Pecten has- 

 tatus 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 inter- 

 rib ; 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 (pi. 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 Rodeo, University of California local- 

 ity 1632. 



PECTEN (PECTEN) CIEEBOENSIS, n. sp. 

 Plate 47, figure 6 



Shell rather small; apical angle about 90°; dorsal edges slightly 

 concave; ventral edge very strongly arcuate. Surface sculptured by 

 fifteen fairly coarse, flat-topped ribs with flat-bottomed interspaces 

 about as wide as the tops of the ribs, interspaces and ribs narrowing 

 rapidly toward the beak, incremental lines inconspicuous. Anterior 

 ear with four fairly heavy radiating ribs ; bysal notch deep ; anterior 

 edge broken ; posterior ear broken. 



The writer has only one specimen of the right valve of this species, 

 but it appears to be so distinct as to be worthy of a specific name and 

 description. 



Dimensions. — Length, 22 mm. ; height, 24 mm. 

 Occurrence. — In the basal beds of the San Pablo Group of the type 

 section. 



