1918 | Packard: Molluscan Fauna from San Francisco Bay 233 
the majority of the entire outside fauna as known from the ‘‘ Alba- 
tross’’ collections are predominately southward ranging. 
This condition may be contrasted with the scareity of southern 
forms within San Francisco Bay. The list of prevalent species for 
the bay (table 2) and for the different divisions of the bay does not 
contain a single predominately southward ranging form. Each of 
these prevalent species is predominately northward ranging. The 
bay fauna as a whole also has a northern aspect. 
There appears to be little if any difference in the distribution of 
the few southern species that live in the bay, although there is a 
considerable seasonal range of temperature in the different divisions 
of the bay. It is perhaps more than a coincidence that the middle 
division of the bay is the richest faunally, and that it is the coldest 
section of the bay during the summer months. 
This distribution of southern forms outside the Golden Gate and 
of northern forms within San Francisco Bay is surprising when it is 
recalled that the water of the bay is relatively warmer in the summer 
than that of the open ocean. In other words, cold summer temper- 
atures do not appear to be an important factor in limiting the north- 
ward range of the southern species. If it can be shown, however, that 
the same species reproduce during the winter months, the conclusion 
would be that the relatively cold water of the bay prevents the de- 
velopment of their larvae, and therefore limits them in their distri- 
bution to the warmer waters of the open ocean. Regardless of the 
value of such a supposition, it is evident that the distribution of the 
southward ranging species is in some way dependent upon the winter 
temperature conditions, and not upon that of the summer. 
The southward range of the northern species might be expected 
to be in some way limited by the warm waters. Such species would 
be expected to comprise the major portion of outside fauna, for the 
summer temperature of the open ocean is lower than that of the bay, 
but such is not the ease. These forms are in the warmer waters of 
the bay. However, they are most abundant in the middle division of 
the bay, which at this season is the coldest portion of the bay. If the 
winter conditions are considered, it is found that the northern species 
are in the colder waters of the bay instead of the warmer water of 
the open ocean. The effect of temperature in limiting these species 
is obscure unless it may be assumed that they reproduce in the winter 
and that the colder bay waters are more suitable for the development 
of their young than the warmer water of the open ocean. Such con- 
