306 University of California Puhlications in Zoology [Vol.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 molluscan 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 pi. 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 in 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 obtained 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 6^/4 

 fathoms ; and the haul was made January 21, 1913. The complete 

 record of this haul is given in table 2. 



