1918] Packard: Molluscan Fauna from San Francisco Bay 219 
The average number of species represented by living specimens 
per station is 6.7. This is not the average per dredge haul, for in the 
majority of cases two hauls were made at a station. Unfortunately 
the author’s records do not give the requisite data upon which to base 
the more accurate average. This number is twice as great as the same 
average for the entire bay and over three times as great as obtains in 
the upper division of the bay, where conditions are least favorable to 
a varied mollusean life. 
The relative abundance of mollusks in the different regions may 
be approximated by taking the average number of lying specimens 
per station. In the list given above of station D 5789, 368 living 
specimens were recorded. These were obtained by dragging a 12-foot 
Agassiz trawl a distance of one mile and then covering the same course 
with a 19-inch boat dredge. It is not presumed that this number of 
living specimens could be considered as representing all of the indi- 
viduals living within the area covered by the dredging operations. 
However, the average of a number of such stations gives a figure that 
has a comparative significance. When the number of living specimens 
only are considered it is found that the average per station for the 
region outside the Golden Gate is 74.4. This is but little larger than 
that for the lower division of the bay (70), but it is about three times 
as large as that for the upper division (26.3). : 
When these figures are compared with the average number per 
> 
dredge haul for the ‘‘quantitative’’ stations of any two of these 
regions, different ratios are obtained. The average number of living 
individuals per haul (orange-peel bucket dredge) for the upper divi- 
sion of the bay is 4, while that for the lower is 27.9. This ratio is only 
about one-half of that obtained above. Obviously these quantitative 
results are the more accurate since they are based upon the actual 
numbers of mollusks living upon equal areas. Thus it appears that 
the figures obtained by the first method can give only a very general 
idea of the relative abundance of the mollusks hying within a given 
area. 
The fauna from San Francisco Bay comprises 81 species and varie- 
ties, 43 of which are pelecypods, 31 gastropods, and 7 chitons. In the 
list of the bay fauna given below those species that were taken exclu- 
sively within the bay are followed by an asterisk. 
Fifty-nine per cent of the species listed below were taken exclu- 
sively within San Francisco Bay. This percentage would be some- 
what decreased had collections been made along the lttoral outside 
