212 University of California Publications in Zoology | Vou. 14 
over the entire bottom was between 0.67 and 0.75 knots per hour. In 
Golden Gate and Raccoon Strait the currents are strong enough to 
scour the bottom, leaving only the coarser sediments. 
The mean annual temperature for the entire bay is 12°91C. The 
highest recorded temperature is 20°6C and the lowest is 6°C. The 
highest of the regional means for the year was obtained in the lower 
division of the bay and the lowest in the middle segment. A seasonal 
range of 12°65C occurs in the northern end of the bay, decreasing to 
4°92C at the Golden Gate and rising to 11°18C at the southern end. 
These extremes of temperature are perhaps of less importance in 
determining the distribution of marine organisms than are those pre- 
vailing at the times of the year that coimecide with the reproductive 
periods of the different species. 
In the above-mentioned report it has been shown that there are 
considerable seasonal variations in the temperatures within the dif- 
ferent regions of the bay. During February the temperatures are 
quite uniform for the entire bay, being at that time lower than are 
those of the ocean outside the Golden Gate. During the latter part 
of April and early May the waters at either end of the bay are warmer 
than in February, whereas those of the middle division are colder than 
they are at the earlier period. At the next period, the latter part of 
July, a rise of temperature is noted, the Golden Gate remaining the 
coolest region of the bay. During this period the temperature of the 
bay is higher than that of the ocean off San Francisco. In the early 
part of October a general decrease in the temperature is evident, and 
at this period, as well as in the early part of May, the oceanic temper- 
atures are nearly the same as those of the bay. In late November a 
general uniformity of temperatures somewhat lower than those of the 
open ocean prevails throughout the bay. The lowest temperatures 
of the year occur in January, at which time the waters of the middle 
division are the warmest, while those of San Pablo Bay are the coldest. 
Both during this month and in February the waters of the bay are 
colder than are those of the ocean. 
From the standpoint of the mollusk, these temperatures may not 
be as Important as are the bottom temperatures. Yet it is not im- 
probable that the temperature of a higher stratum is more significant 
in determining the distribution of the mollusk, for it is in the higher 
stratum that the larval stages are generally passed. The annual 
range of the bottom temperature for the entire bay is 8°35C. The 
bottom and surface temperatures are more nearly alike in the winter 
than in the summer. 
