1918 | Packard: Molluscan Fauna from San Francisco Bay 221 
The more common or prevalent species may arbitrarily be defined 
as those that were taken at one-fourth or more of the stations of any 
given group of stations, as suggested by Sumner ef al. (1913, p. 69). 
In the following list (table 2) the prevalent species for the different 
divisions of San Francisco Bay are given. 
TABLE 2 
PREVALENT SPECIES FOR THE ENTIRE Bay (QUANTITATIVE HAULS) AND FOR EACH 
OF THE THREE DIVISIONS Hntine Bay 
(quantita- 
Prevalent species Upper Lower Middle tive hauls) 
Cardium ecorbis ..... rae x x x 
IMiacom ab alli nie aise snses cere eaneeereee x x 
INTE KOC aR TAK ( TWIG es ee eee ae shes x x 
Macoma nasuta .- x x x 
Mya arenaria x x 
Mya californica .. g x 
Mytilus edulis -. x fat 
Ostrea lurida x x 
Paphia staminea 
Schizothaerus nuttalli - Pass ne x 
Mibhvayr Salann Cll OS akecesqcoceccesceec esc 2h es x x 
JINGLE G69) 0) Oak) es cen ans er ane 22 x x 
The distribution of these prevalent species, together with six others 
that oceurred at ten or more stations, has been plotted upon outline 
maps of San Francisco Bay. Localities at which a particular species 
was taken alive are indicated on the plates (42-55) by a dot, while 
those at which only shells were taken are indicated by a cirele. These 
charts show at a glance the approximate ranges of the species within 
the bay. It will be noted that the distribution patterns differ quite 
widely. 
The most productive station within San Francisco Bay from the 
standpoint of the number of species is D 5796, situated north of Angel 
Island within Raccoon Strait. The species listed on the following page 
were obtained at this locality. 
This list is not entirely comparable to that given in table 1 of 
the most productive outside station, for the largest number of deter- 
mined species and the largest number represented by living specimens 
were not here obtained at the same station. At D 5781 twelve different 
living specimens were dredged. 
A study of the ranges of the known fauna as listed in the catalogue 
of species gives some suggestion as to the position of the fauna in 
relation to climatie zones. One hundred and twenty-nine species and 
