304 University of California Publications in Zoology (Vou. 13 
in 26, or 56 per cent of the total. M. balthica and M. nasuta were 
taken together alive in three hauls. Shells of these two species were 
associated at two additional hauls. Specimens of M. balthica and 
M. inquinata were found together in three dredge hauls. Shells of 
M. nasuta and M. inquinata were taken together at 11 hauls. Speci- 
mens of all three species were taken at two localities. It is certain, 
then, that the more distantly related species M. balthica and M. nasuta 
live together within the restricted area covered by the jaws of the 
orange-peel bucket dredge, and it is probable that the more closely 
related species M. nasuta and M. inquinata may occur together within 
the same restricted area. Attention should, however, be called to the 
different distribution pattern of these three species resulting from 
the plotting of all of the known local occurrences upon outline maps 
of San Francisco Bay. Such a procedure shows that M. inquinata 
occurs almost exclusively in the middle division of the bay, whereas 
M. balthica and M. nasuta have a much more general distribution. 
The quantitative hauls are too few in number to serve as a basis 
for conclusions regarding the areal distribution of any of the species. 
Therefore such studies are reserved for a fuller treatment im another 
paper. 
The most common or the prevalent species of the quantitative hauls 
may be defined as those that occur at one-fourth or more of the hauls 
(See Sumner, Osborn, Cole, and Davis, 1913, p. 69). 
This list of prevalent species is as follows: 
Carding con bis ieee eee 19 hauls 
Macoma balthiea ...... ..-12 hauls 
Macoma inquinata .... ..-13 hauls 
Macoma nasuta .... ...-26 hauls 
Mya arenaria ......... .-17 hauls 
Mya californica - .-28 hauls 
Mytilus edulis -.. ...18 hauls 
Ostrea lurida ..... 14 hauls 
Zirfaea gabbi ..... 13 hauls 
Mh aislLarm ell OS ae eae ene ene 15 hauls 
Of these, only Mya california and Macoma nasuta were taken alive 
more than ten times. 
These prevalent species include the most adaptable forms found 
in the local fauna. Most of them are distributed quite uniformly 
throughout the bay, being able to withstand easily the extremes of 
the diverse environments found within these waters. The hardiness 
of these species is attested also by their wide geographic distribution, 
showing a marked range of environmental conditions. It is not sur- 
