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 smialler shells, which are but sparsely repre- 

 sented in the fauna. Hence in the case of such forms as Turhonilla, 

 Odostomia, or MelaneUa 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 



Physical Characters op San Francisco Bay 



Sumner et at. (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 

 lies 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. 



