1918] Packard: Molluscan Fauna from San Francisco Bay 237 
Group 1—Mud: not less than 90 per cent of mud. 
Group 2—Mud and sand: not less than 50 per cent of mud or 
10 per cent of sand. 
Group 3—Sand: not less than 90 per cent of sand. 
Group 4—Sand and mud: not less than 50 per cent of sand or 
10 per cent of mud. 
Group 5—Sand and gravel: not less than 50 per cent of sand or 
10 per cent of gravel. 
Group 6—Gravel, shells, or stone: total of these equalling not less 
than 90 per cent. 
Group 7—Gravel and sand: not less than 50 per cent of gravel 
or 10 per cent of sand. 
In a few instances where the percentage of a certain type of 
material at a station lies within 1 or 2 per cent of a certain group to 
which it logically belongs it has arbitrarily been included in that 
group. Such cases are indicated in the list below by means of an 
asterisk. In this list the dredging stations, or their equivalent when 
necessary, of the localities at which precise bottom data are available, 
are given. The number in the right hand column refers to the bottom 
group to which the station belongs. 
These stations are quite uniformly distributed throughout the bay, 
and may therefore be considered as representative of the bottom con- 
ditions prevailing within San Francisco Bay. 
Upon the basis of the above groupings (table 6), it may be caleu- 
lated that at 18 per cent of the 72 stations mud (group 1) was encoun- 
tered, while at 12.5 per cent sand (group 3) was found, and at 9.7 
per cent of the stations gravel and stones (group 6) prevailed. If 
groups 1 and 2 are considered together it is found that the bottom was 
prevailingly muddy at 43 per cent of these stations, prevailingly sandy 
at 44.3 per cent, and gravelly at 12.4 per cent. It is of interest to 
consider the character of the bottom by regions. In San Pablo Bay 
a prevailingly muddy bottom occurs at 53.3 per cent of these stations 
within that region and a predominately sandy one (groups 3, 4, and 5) 
at 46.6 per cent. Mud was encountered at 33.3 per cent and sand 
at 6.6 per cent. These figures may be compared with those for the 
lower division of the bay. In that region a prevailingly muddy bottom 
oceurs at 77.6 per cent and a predominately sandy one at 22.2 per 
cent of the stations. Pure mud occurred at 7 out of the 18 stations 
within this region, but at none of the stations was the bottom com- 
