1918] Packard: Quantitative Analysis of Molluscan Fauna 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 RELATIVE ABUNDANCE OF MOLLUSCAN SPECIES AND INDIVIDUALS FOR THE 
DIFFERENT SECTIONS OF SAN FRANCISCO Bay 
5 3 3 
rp ee ae ae ese SS 
nD 5 na 
go) ge Ree) ga) (ieee BE ee. Se 
os 22 255 98 oe a2 an ae ag 
Hee Sas aie: oe GHeueSs CHS (em, 7a 
53 BR sia 5.4 ER 25 Sry Be oS 
aa Fiph. Rinieo) “Bey ACH 23K Hom Hoa) vo 
Om o ond wus of2 one ont one on 
woo ws wO5 was wns wot wot wos be 
S35 So S55 #7 aod Sef ash ago a4 
52 Be SO Bon 1 Bsik Ben Hon Hon eS 
baa ee San P22 222 fue Sao Pav Ps 
ga Fm Fes Fan bon Fon baa ben ba 
Entire bay .............. a9 c4 LS 454° “134:8) 180:3. 48 1.06 8.6 
Upper bay ...... Serene easly les 4.08 17.4 21.5 2.2 08 7.8 
Middle bay .- = (oe 400 - 2:4, <80:0) e235:9" 8315.9 ‘Gil 2.2 9.9 
Lower bay .....-:--:-::--+ 6:9) *G:20 21:2) 27:9) WOOF NOVO: 15:2 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 mollusean 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- 
