1918] Packard: Quantitative Analysis of Molluscan Fau7ia 317 



FACTORS GOVERNING THE DISTRIBUTION OF THE 

 MOLLUSCA 



The previous sections of this paper dealt with the general distri- 

 bution of the Mollusca and with the actual numbers of individuals 

 living within a definite area. It now remains to investigate the dis- 

 tribution of the mollusks from the standpoint of their environment. 

 Some of these governing distribution are : the physical character of 

 the bottom ; the salinity, temperature and depth of the water ; the 

 distribution of the plankton which serves as food ; and other biotic 

 factors including other organisms which may not be beneficial to the 

 animal under consideration. 



The limited number of quantitative hauls offers less conclusive 

 evidence regarding the importance of some of these factors than do 

 the more numerous qualitative dredge hauls made during the general 

 survey of the bay. Petersen (1913, p. 5) has shown that the common 

 dredge gives an entirely different picture of the benthos from that 

 obtained by means of the quantitative type of dredge. The latter 

 brings to the surface not only the organic matter from that locality 

 but also the bottom materials upon which or in which the animals lived, 

 thus giving a more correct idea of certain factors of their environment. 



TABLE 6 



The Eelative Abundance op Molluscan Species and Individuals for the 

 Different Sections of San Francisco Bay 



T3 M » 'I "I 



^ ^ '^^a NH ^ .2 "" o "" Q, 



o o o-^a^o Oco g oP<og' CO 



SS S§ ^go Sis S3 c« Sii S-S °o 



^u §t sS-g- «-=< §■> Js 2=^ S "> P-ll 



3 © 3 S 3 o) «= S-- s.S ^T3 ^^ s^ 0)4; 



op, Rp, fis^tc C.^_ Cx!— -S-r— Co^ ^ ^-^ "^ a 



a> m »is O'S^ <D'^2 "li'-'S »--3 <»co2 ijw2 *'—' 



cS'3 cS (P oj o-.; ca"->G c3tS"^ =8 R-^ <s'S~ s'S-^ ^" 



Sh5< y-' ^ u Qj^-* i~< Q> u Shcsjh ^'"^ ^0)^ ^-'O'^t !r!3 



go, S 53 £P.;>» ®>S g (L 0, ?>«<D Sp.1' SftS g =« 



<!-<<; <! <! <! ■< -fi < 



Entire bay 5.9 5.4 1.8 45.4 134.8 180.3 4.8 1.06 8.6 



Upper bay 2.3 2.1 1.3 4.08 17.4 21.5 2.2 .08 7.8 



Middle bay 7.5 7.0 2.4 80.0 235.9 315.9 6.1 2.2 9.9 



Lower bay 6.9 6.2 1.2 27.9 79.0 107.0 5.2 1.6 8.3 



A tabulation of data derived from table 1 shows in table 6 that 

 the conditions within the different divisions of San Francisco Bay 

 are not equally favorable to molluscan life. This analysis of these 

 more restricted areas clearly shows that the middle division of the 

 bay is a much more favorable habitat for the mollusk than either of 

 the other two divisions. The lower section is much richer per haul 

 than the upper in every respect, the average number of living indi- 



