242 University of California Publications in Zoology (Vou. 14 
organisms. Its capacity is 2.5 cubic feet, and it encloses a circular 
area of 7.8 square feet. 
The material collected by means of this apparatus was carefully 
sorted and all of the macroscopic organisms were preserved in form- 
alin or alcohol. The record of the molluscan material include the 
following items: the name of the species, the number of individuals 
of each species, the condition of each specimen at the time of dredging, 
and the maximum, minimum, and modal lengths of each species from 
each haul. 
The fauna obtained in this manner comprises 23 pelecypods and 
12 gastropods. This number represents about 43 per cent of the bay 
fauna. 
The list of prevalent species given on page 221 includes the most 
adaptable forms found in the local fauna. These species have a gen- 
eral distribution throughout the bay, bemg also conspicuous because 
of the wide geographic ranges, which in several instances are circum- 
polar. 
It has been shown in another paper (Packard, 1918) that the aver- 
age quantitative haul is richer in bivalves (4.8) than in univalves 
(1.0). On the average 45.4 living mollusks occur within the area coy- 
ered by the jaws of the orange-peel bucket dredge. The most produc- 
tive haul yielded 16 species, 4 of which were represented by living 
specimens. The relative abundance of molluscan species and indi- 
viduals for the different divisions of the bay was found to be 2.3 species 
per haul in the upper bay, 7.5 in the middle, and 6.9 in the lower, or 
91.5 individuals per haul in the upper, 315 in the middle, and 107.5 in 
the lower bay. 
The mollusks obtained in the quantitative hauls show little rela- 
tionship in distribution to depth, although the average number of 
living individuals is greater in the deeper portion of the bay. How- 
ever, this has little significance, since the deeper hauls were made 
within the middle division of the bay, where this apparent relation- 
ship may be accredited to other factors. The character of the bottom 
is an important factor in distribution. It can be shown that the 
number of individuals per unit area is largely dependent upon the 
particular type of bottom. A bottom characterized by sand and shells 
yields on an average 174 individuals, while a bottom of mud and shells 
yields only 82.5. This conclusion does not harmonize with the one 
given above (p. 240) that the muddy bottom supports the larger num- 
ber of mollusks. This discrepancy may be due to the trawl dredge 
not sinking deep enough in sand to capture a representative fauna. 
