With 2 figures and 6 tables.
Introduction.
Investigations to determine the cause of shellfish poison have been conducted during the past several years in these lahoratories. A complete list of the publications concerned with this poison is included in the bibliography. The purpose of this report is to present the quantitative and qualitative distribution of phytoplankton in the Sail Francisco region as shown by the sea water samples collected during one phase of these investigations which includes the period from June 17, 1932 to June 17, 1935. The relation of phytoplankton to the poison has been reported elsewhere (S o m m e r , W h e d o n , K 0 f 0 i d and S t o h l e r ) . The collection of samples was greatly facilitated by the use of a pier kindly offered for this purpose by the Olympic Saltwater Company of San Francisco, California.
Methods.
A preliminary study of the feeding habits of the mussel, illytilus californianus Conrad, during the earlier phases of the problem of shellfish poison, led t o the assumption that an average of nineteen liters of sea water were filtered twice daily by each mussel. This quantity of water was taken, therefore, as the standard sample to be filtered during each collection. Ordinarily, the collections were made shortly before high tide or soon after in order to ascertain as accurately as possible the quantity and quality of phytoplankton available for food at the time when the mussels were feeding. Difficulty was experienced in collecting at'times when the tide level was greater than four feet