1918] Packard: Quantitative Analysis of Molluscan Fauna 301 
lined above. This method of procedure when large masses of finely 
comminuted shells were concerned is not above criticism, yet it seemed 
to be the most practical method of treatment available. 
Such a method gives excellent results for the larger mollusks and 
a fair representation for the smaller ones that are abundant, but it 
is slightly inaccurate for smaller shells, which are but sparsely repre- 
sented in the fauna. Hence in the case of such forms as Turbonilla, 
Odostomia, or Melanella the figures given do not represent the actual 
numbers taken at a station. 
The record based upon these methods shows for each species at 
each dredge haul the name, the number of individuals of each species 
(often approximate when these numbers were large), the conditions 
of the specimens at the time of dredging, and the maximum, minimum, 
and the modal lengths. 
GENERAL DISTRIBUTION OF THE MOLLUSCA 
PuHysicaL CHARACTERS OF SAN FRANciIscO Bay 
Sumner et al. (1914, p. 22) recognized three physical and biolog- 
ical divisions of San Francisco Bay. The ‘‘upper’’ portion includes 
San Pablo Bay, the ‘‘middle’’ one extends from a line passing through 
the points of San Pedro and San Pablo to a line drawn from the 
Ferry Building to the Goat Island Light ; the third or ‘‘lower’’ division 
hes south of the latter line. 
The quantitative stations comprise forty-three hauls made with 
the orange-peel bucket dredge between the dates of December 9, 1912, 
and February 3, 1913. They were distributed from a point near the 
southern extremity of San Francisco Bay to Carquinez Strait. In 
the upper division of the bay twelve hauls were made at stations 
D 5815 to D 5820 inclusive. Twenty hauls at stations D 5821 to D 5830 
inclusive were made within the middle portion; while eleven were 
made in the lower section of the bay at stations D 5831 to D 5841 
inclusive. The position of these stations may be seen by referring to 
plate 12. 
The physical conditions at these representative stations of San 
Francisco Bay are influenced largely by the surrounding topography. 
The Sacramento and San Joaquin rivers and several intermittent 
streams contribute considerable volumes of water to the bay. The 
total discharge of these streams affects the temperature and the salinity 
of the bay, besides bringing in sediment that is in part deposited 
within that basin. 
