1918] Packard: Molluscan Fauna from San Francisco Bay 229 
strongly with the wide mud flats in the vicinity of the estuaries and 
in front of the lowlands. These areas are subject to the same extremes 
of temperature as the adjacent land, whenever the tide is out. The 
life at one time may be bathed in fresh water and at another covered 
by salt water. Unfortunately these factors have not been investigated 
by this Survey. 
There has been no attempt to make a complete biological survey 
of the intertidal region, although it covers 13.6 per cent of the bay 
(Sumner ef al., 1914, p. 20). The collecting stations mentioned above 
were for the most part on rocky shores. <A notable exception to this 
is the Key Route Pier (Oakland) locality, where specimens were taken 
from the piles and the mud flats near by. Along the rocky shores at 
the high-tide mark the two species of Littorina and the various mem- 
bers of the Acmaeidae are the most conspicuous mollusks. Those 
localities, as in the case of the flats bordering the marsh lands, are 
lacking in suitable objects for support, and abound in specimens of 
Cerithidea californica. Farther down the beach, near the low tide 
mark, several of the burrowing clams occur in the sandy or muddy 
localities, while Thais and the chitons occupy a similar position on the 
rocky beaches. 
The following species were taken by the Survey only at the shore 
stations: Acmaea patina, Lacuna unifasciata, Littorina planazis, 
Mopalia kennerlyi var. swani, Saxicava pholadis, Tegula funebrale, 
Tomcella lineata, Trachydermon: dentiens, and T. raymondi. It is 
probable that these species are not restricted to the littoral zone, but 
that they represent a few of the more characteristic forms of that 
region. The fauna of the littoral, of course, includes a considerable 
number of species that range into the adlittoral zone. 
The laminarian zone, within San Francisco Bay, is not divisible 
into definite subzones upon the basis of the molluscan life. However, 
it is of interest to note a few of the species that were dredged exclu- 
sively in the shallower portion of the bay. In no ease is it possible 
to state that the depth factor is responsible for this apparent distri- 
bution. An area equal to about three-fourths that of the entire bay 
is comprised within the limits of the low-tide mark and that of the six 
fathom line. Out of the 47 species that were taken by the Survey at 
depths ranging from 0 to 6 fathoms, only five were restricted to that 
depth. These are: Anomia peruviana, Arca transversa, Calliostoma 
cossatum, Margarites pulloides, and Murex interfossa. All of these 
were rarely dredged, so in their records have little significance. 
