226 University of California Publications in Zoology — [Vou. 14 
DISTRIBUTION OF THE MOLLUSKS 
GENERAL DISTRIBUTION 
Our knowledge of the benthos is largely derived from various 
types of dredgings. Under the ordinary methods a dredge is hauled 
along the bottom for a distance often measured in miles. Obviously 
such a haul may traverse a diversified bottom and the fauna from 
several types of bottoms becomes mixed. Such a commingling has to 
some extent been minimized in this survey by making the dredge hauls 
in the majority of cases less than a mile in length. The different 
environmental conditions of San Francisco Bay are now accurately 
known from the published analyses of the bottom samples and the 
other hydrographic data obtained by the Survey. 
A system of dredging that offers greater possibilities for biological 
purposes has been employed by Peterson (1914, p. 5) within the 
Cattegat, and still another by Sumner in this Survey, whereby a 
numerical census of the life of a small area was obtained. The orange- 
peel bucket dredge (see Sumner et al., 1914, p. 7) had not previously 
been used for biological work. It is evident that ‘‘its chief advantage 
hes in the taking of comparatively large masses of mud from a single 
spot, and particularly in the penetrating power of the apparatus, 
which renders possible the capture of deeply burrowing annelids, 
lamellibranchs, ete.’’ The results of this phase of the Survey are 
summarized below. The occurrences of the species taken by means 
of this dredge are given in the eatalogue of species, and these data 
have been used in the plotting of the distribution charts (pls. 42-59). 
In order to analyze the environment of a species, it is necessary to 
consider the factors that in any way might react with the organism, 
causing it during its own life or during a number of generations to 
better adapt itself to its surroundings. From the standpoint of the 
Mollusea, these factors are numerous and little understood. The ecol- 
ogist is generally unable to trace the processes of adjustment of a 
species to its environment as the palaeontologist apparently can do 
from a study of successive horizons. He must, therefore, content him- 
self with observations upon the degree of perfection of the adjustment 
between the animal and its environment at a definite period of time. 
If a bivalve is modified, for instance, according to one biological 
theory, as a result of the impact of its environment or through selective 
