1918] Packard: Molluscan Fauna from San Francisco Bay 213 
The salinity of San Francisco Bay ranges from 3.25 to 33.27 per 
mille. The mean for the entire bay for the year was found to be 
27.48. The regional annual mean is less than 16 per mille in Car- 
quinez Strait, while it reaches as high as 31 just within the Golden 
Gate. As might be expected, the seasonal range is greatest in San 
Pablo Bay, which receives the discharge of the Sacramento and San 
Joaquin rivers, reaching a minimum at the Golden Gate and increasing 
but slightly toward the lower end of the bay. The seasonal minimum 
mean salinity for the entire bay occurs in April and May and the 
maximum in Oetober, according in general with the high and low 
water stages of the Sacramento and San Joaquin rivers. The bottom 
salinity for the entire bay is greater than the mean surface salinity 
throughout the year, the difference between the two being greater 
during April and May, when the surface salinity is the lowest for the 
year. ; 
An examination of the bottom conditions of the bay reveals a 
diversified character such as might be expected by one familiar with 
the geographic features of this region. Materials ranging from large 
angular stones to fine muds are represented. San Pablo Bay is muddy 
except for a small area at the lower end of Carquinez Strait. Mud 
oceurs also in local areas within the middle division, but in the deeper 
water explored by the Survey the bottom is arenaceous. The bottom 
within the Golden Gate is composed in the main of sand and gravel, 
but in places the currents are so strong that they prevent the accum- 
ulation of little except gravel. The lower division of the bay is pre- 
dominantly muddy. At some localities within the lower and middle 
segments the bottom appears to be paved with shells of Ostrea lurida 
and Mya arenaria. The distribution of the different types of bottoms 
is admirably shown by Sumner ef al. (pl. 5). 
Chemical analyses have been made of bottom samples from a 
number of stations. They show that the percentage of free or com- 
bined nitrogen is higher in the upper and lower divisions of the bay. 
Biological—The different biological environments under which the 
mollusks of San Francisco Bay are living cannot be definitely de- 
scribed. The materials obtained by the operations of the ‘‘ Albatross”’ 
are. now being studied by specialists. When the different reports are 
completed it may be possible to recognize certain relationships be- 
tween the distribution of some of the mollusks and that of other groups 
of animals or plants. The relative abundance of the plankton, which 
probably serves as the most important food supply of the peleeypods, 
