306 Unwersity of California Publications in Zoology (Vou. 13 
Table 1 shows that the average number of species of Pelecypoda 
per dredge haul is 4.83 as compared with 1.06 of Gastropoda. This 
preponderance of bivalves may be characteristic of inclosed waters, 
for it is considerably less, judging from the qualitative hauls alone, 
in the open waters just outside of the Golden Gate. The relative 
abundance of the forms is shown in the fourth column, where it is 
found that 45.59 living specimens were taken in the average quanti- 
tative haul representing the mollusecan population of 7.8 square feet 
of bottom. These living specimens represent 1.85 species, showing 
that as a rule but a comparatively few forms live together at the 
same time within an area less than eight square feet. The largest 
number of living individuals dredged from a single locality is 458 at 
station D 5828 A, which is within the middle division of the bay just 
east of Angel Island (see pl. 12). 
The number of dead shells, representing as they do the accumu- 
lation of a considerable period of time, have but little interest in a 
faunistic study. The shells, which are often heaped into veritable 
banks, may be transported by currents or various marine animals, of 
which the hermit crab is the most important. At certain localities 
the dredge was often completely filled with old valves of Ostrea lurida 
or Mya arenaria. Occasionally these hauls contained no living speci- 
mens of the species so abundantly represented by dead shells. This 
suggests a recent change in the physical conditions, at least im quiet 
waters, of such a nature as to be detrimental to that species. It is 
not improbable that the molluscan fauna of the bay is undergoing 
modifications due to the close proximity of the cities around the bay. 
The average number of dead shells per dredge haul is 134.8, which is 
far under the actual number that would be obtaimed if several of the 
above mentioned hauls of oyster shells had not been omitted. 
Station D 5833 is the richest faunally of all the quantitative hauls. 
This most productive haul was made 0.3 of a mile west of the Oakland 
Harbor Light, within the lower division of the bay. The bottom was 
characterized by Sumner et al. (1914, p. 190) as being composed of 
90.5 per cent of mud and 9.4 per cent of sand; the depth is 614 
fathoms; and the haul was made January 21, 1913. The complete 
record of this haul is given in table 2. 
