308 University of California Publications in Zoology L^ol. 13 



This unit area within the upper division of the bay would yield 

 two species, judging from the average number of species per dredge 

 haul for that region. Similar averages for that portion of the bay 

 indicate that such an area would support four living specimens and 

 seventeen old shells. 



The same area within the middle division would yield, according 

 to the same line of reasoning, seven species, while the living individuals 

 would number 80 and the old shells 235. 



A similar area within the lower division would appear to yield six 

 species, twenty-seven individuals and seventy-nine old shells. 



The particular species represented within these three hypothetical 

 areas can not be determined. It is probable that such an area de- 

 picting the average conditions would contain some of those species 

 that have been listed as the prevalent species for the region considered. 

 The commonest simple combination of species for the upper division, 

 for instance, would be the two species most frequently dredged within 

 that region, but such a combination out of a number of other possible 

 combinations would rarely be obtained. 



This difference in the abundance of the molluscan life within the 

 different regions of the bay is shown in plate 13, where the circles of 

 different sizes stand for the different species and the number of circles 

 for the number of living individuals obtained in the average dredge 

 haul for the designated divisions. No attempt has been made to show 

 the number of old shells. 



ECONOMIC CONSIDERATIONS 



The molluscan fauna of San Francisco Bay and environs includes 

 a number of edible peleeypods. The two local species most commonly 

 found in the markets of the Bay region are Mya arenaria, the ' ' soft- 

 shelled, " "mud" or "eastern clam," and Paphia staminea, the "hard- 

 shell," or "butter clam." Other well known northern clams that 

 occur in the vicinity of San Francisco include : Saxidomus nuttalli, 

 Schizothaerus nuttalli, Mytilus edulis, Mytilus californicus, Siliqua 

 nuttalli, Ostrea elongata, Ostrea lurida, Panope generosa, Cardium 

 corhis, and Pholadidea penita. Two other Californian species, Tivila 

 crassatelloides and Chione undatella, are frequently seen in the San 

 Francisco markets, but they are southern species, the former, the Pismo 

 clam, coming principally from San Luis Obispo County. 



Mya arenaria is predominantly a mud-dwelling species, and occurs 



