304 University of California Publications in Zoology [^ol. 13 



ill 26, or 56 per cent of the total. 31. halthica and M. nasuta were 

 taken together alive in three hauls. Shells of these two species were 

 associated at two additional hauls. Specimens of M. halthica 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. halthica 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 31. inquinata 

 occurs almost exclusively in the middle division of the bay, whereas 

 31. halthica and 31. 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 in 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 : 



Cardium corbis 19 hauls 



Macoma balthica 12 hauls 



Macoma inquinata l-^ hauls 



Macoma nasuta 26 hauls 



Mya arenaria 17 hauls 



Mya californica 28 hauls 



Mytilus edulis 18 hauls 



Ostrea lurirla - 14 hauls 



Zirfaea gabbi 13 hauls 



Thais lamellosa 15 hauls 



Of these, only 3Iya 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- 



