234 University of California Publications in Zoology (Vou. 14 
clusions as to the breeding habits of the mollusean species have little 
value, unless it be that they stimulate much needed investigation along 
these lines. 
The occurrence of the predominately southward ranging species 
in the open ocean and their general absence from San Francisco Bay 
indicates that their distribution is in some way determined by the 
winter temperature. It is to be presumed that temperature acts in 
some way upon the reproductive activities of the mollusks. On the 
other hand, the northern species occur more abundantly within the 
bay, which during the winter months is relatively colder than the 
open ocean. This suggests that the warm water of the winter in some 
way limits the southern range of the northern species. 
INFLUENCE OF SALINITY 
Salinity is especially significant as a factor in determining the 
distribution of marine invertebrates in such regions as estuaries or 
salt marshes. The Sacramento and San Joaquin rivers reduce the 
salinity of the Carquinez Strait at the upper end of San Pablo Bay 
to an annual mean of less than 16 per mille. In this same region at 
hydrographic station H5975 the seasonal range of bottom salinity 
lies between 13.35 and 19.14 per mille. Nevertheless this low salinity 
does not impose an effective barrier to Macoma balthica, M. nasuta, 
Mya arenaria, nor Mytilus edulis. Still farther up the stream at 
stations D 5760 and D 5761 the conditions are unfavorable to even 
these hardy species. They are likewise absent at D 5759, at the mouth 
of Napa Creek. It is evident that in these cases it is the minimum 
salt concentration that determines the distribution of these mollusks. 
Estuarine conditions exist also at station D 5766, within Alameda 
Channel. Since but little fresh water empties into this inlet, the sa- 
linity is presumably much the same as at the nearest hydrographic 
station, H 5008, which is typical for that portion of the lower division 
of the bay. It is not surprising to find that the fauna from that inlet 
is also typical of the lower bay, and that it includes species not dredged 
within Carquinez Strait. 
These few species that are capable of living in water of such low 
salinity cannot be properly designated as brackish water forms, for 
they occur abundantly in other regions where the salinity is high. 
All excepting Mya arenaria are prevalent species within the middle 
division of the bay, where the mean annual salinity is nearly twice 
as great as it is in Carquinez Strait. These species represent the more 
