322 University of California Publications in Zoology [Vol. 13 



Group 7. Sand and shells 



Living: Mya calif ornica 



Macoma balthica 



Epitonium hindsi 

 Dead: Mya California 



Paphia staniinea 



Epitonium hindsi 



Nassa fossata 



Nassa perpinguis 



Thais lamellosa 



The above list shows several different associations of species. Of 

 the prevalent species, Cardium cordis, Macoma nasuta, Mya arenaria, 

 and Zirfaea gahhi appear to be predominantly mud-dwelling forms; 

 while Mya californica, Macoma 'balthica, M. inquinata, Ostrea lurida, 

 and Thais lamellosa may be classed as sand dwellers. Although these 

 conclusions are tentative, because of the paucity of the hauls upon 

 which they are based, they suggest the broad features of the different 

 molluscan communities. 



Relation to Salinity 



In order to determine the influence of salinity upon molluscan 

 distribution, a comparison of a curve showing the number of living 

 mollusks for most of the quantitative stations with salinity curves for 

 the equivalent hydrographic stations as published by Sumner et al. 

 (1914) may be made. In these curves the stations are arranged along 

 the horizontal axis, at distances proportionate to their relative positions 

 in the bay. The average number of living mollusks from the several 

 hauls made in the immediate vicinity of the hydrographic stations is 

 represented along the vertical axis of the specimen curve. 



There is apparently little correspondence between the areal density 

 of the mollusks and the mean annual salinity. This is evident by 

 referring to figure B. 



It appears, however, that the mean annual salinity at stations 

 D 5815 to D 5820 inclusive (left end of curve) is unfavorable to an 

 abundant molluscan life. The specimen curve as well as the following 

 table indicates that the average number of individuals per haul is 

 greatest for those stations having a mean annual salinity between 28 

 and 30 per mille. 



