308 University of California Publications in Zoology (Vou. 18 
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 hving 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 pelecypods. 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 
oceur in the vicinity of San Francisco include: Saxidomus nuttalli, 
Schizothaerus nuttalh, Mytilus edulis, Mytilus californicus, Siliqua 
nuttalh, Ostrea elongata, Ostrea lurida, Panope generosa, Oardium 
corbis, 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 
