228 University of California Publications in Zoology (Vou. 14 
for not only does the intertidal species Coluwmbella gausapata live 
under such conditions but Nassa perpinguis, another shallow water 
form, was dredged from a depth twelve times as great at the station 
west of the Farallon Islands. Temperature in general decreases with 
depth, but within the comparatively shallow waters of San Francisco 
Bay other factors interfere in such a way as to cause the average 
mean bottom temperature during a portion of the year to exceed that 
of the surface by 0°2C. 
Light penetrates oceanic waters to a depth of about 100 fathoms 
sufficiently to assist in the metabolism of plants. The components of 
sunlight do not all reach such depths. Some, like the red rays, are 
thought to be effective but little below 7 fathoms, while the blue and 
green rays extend to much greater depths. Engelmann (1883) and 
others maintain that the quality of light determines the distribution 
of green, brown, and red algae. Light becomes a factor in determining 
the distribution in depth of any mollusk living directly or indirectly 
upon a particular type of algae. However, lack of data regarding the 
distribution of the algae within the loeal waters prevents the further 
consideration of this matter. 
Dissolved gases play a conspicuous réle in the life of the benthos. 
The amount of gas held in solution in sea water is in a large measure 
a function of the pressure. Tidal currents, of course, prevent the 
stagnation of any considerable portion of a water body. As a result 
of such movements of the water an essential gas, as oxygen, becomes 
distributed even within the depths. In the same manner a harmful 
vas such as hydrogen sulphide, generated from the decay of organic 
matter, is prevented from accumulating. Although we have no direct 
evidence, it is probable that the mean rate of water-flow over the 
bottom of the bay, as computed by Sumner ef al. (1914, p. 28), is 
sufficiently high, so that neither the lack of oxygen nor the excess of 
a harmful gas has much significance to the mollusk living within San 
Francisco Bay, except locally near the mouths of sewers. 
Three bathymetric zones are recognized within the region covered 
by this report. They are the littoral, or intertidal, the laminarian, 
including practically all of the ‘‘Albatross’’ dredging stations, and 
the deep water zone represented by a single haul made in 815 fathoms 
off the continental shelf west of the Farallon Islands. 
The littoral zone of San Francisco Bay varies considerably as to 
physical conditions. The narrow, rocky benches bordering a portion 
of the Golden Gate and many of the islands of the bay, contrast 
