50 University of California Publications in Zoology [ Vou. 14 
where it attains much higher temperatures in summer and much lower 
ones in winter than would waters confined to the more temperate coastal 
belt. Our studies of salinity for one complete annual cycle show that 
on the average about nineteen per cent of the total volume of water 
which was contained in the bay at any given time during the period 
had been derived from these rivers. 
(2) The ocean waters of this portion of the Pacifie Coast, in conse- 
quence of hydrographic conditions which eannot here be discussed, 
undergo a comparatively slight annual temperature range (see Hol- 
way, 1905, Thorade, 1909, and McEwen, 1912). According to Thorade, 
that part of the ocean which adjoins San Francisco Bay lies between 
the isotherms of 11° and 12° C in February and March, when the water 
is coolest, and between the isotherms of 14° and 15° C in October, when 
it is warmest. Causes peculiar to the western coasts of certain conti- 
nents prevent these waters from attaining the summer temperatures 
which are proper to their latitude. Sinee, as has been shown above, 
about one-sixth of the entire contents of the bay is discharged during 
the average ebb tide, to be replaced by water from outside during the 
ensuing flood, it is inevitable that the local ocean temperature should 
exert a marked influence upon that of San Francisco Bay. 
A comparison is none the less interesting between the water temper- 
ature of San Francisco Bay and the air temperature of points in the 
vicinity. For the twelve months commencing February 1, 1912, the 
mean air temperatures of San Francisco and Berkeley, on opposite 
sides of the bay, were 13°44 C and 13°62 C, respectively, according to 
United States Weather Bureau records. The mean of these two figures 
is 13953 C. The mean temperature of the bay during this period was 
12°91 C, or 0°62 lower than that for the air. 
The curves in figure H permit of a comparison between air and 
water temperatures throughout the year. The curve for water tem- 
perature is similar to those already shown in figures D and E, being 
in the present case based upon the mean of surface and bottom figures. 
The air temperatures for the month immediately preceding our earliest 
hydrographie observations have here been included. In comparing 
these curves it must be borne in mind that that for the water is based 
upon observations at intervals of two months, while those for the air 
are based upon monthly means. 
It will be seen that during the colder months the curves for the 
air lie below those for the water, the converse being true, on the 
whole, during the warmer season. These conditions are better shown 
