1920] Allen: Plankton of the San Joaquin River 27 



The whole series of collections for 1913 is now (1916) in the per- 

 manent possession of the Department of Zoology of the University of 

 California. 



ORGANISMS FOUND IN SAN JOAQUIN PLANKTON 



Definitions 



Professor Kofoid's definitions are followed as closely as possible, 

 although only a few terms will be used. This report designates only 

 three types of planktons, i.e., the continuous, the periodic and the 

 adventitious. No closer distinction is advisable in view of the writer's 

 lack of definite knowledge of life histories of various species. There 

 has been no difficulty about the application of the general term 

 "plankton" to the typically mixed population of minute living 

 things found in these collections, because there was no case observed 

 in which the plant or animal seemed distinctly out of place. It might 

 be rare, and perhaps evidently ill-fitting, but in no way could its 

 presence be regarded as surprising. Hence it becomes perfectly 

 natural to apply the term collectively to all the organisms found. 



Component Forms 



A total of 471 planktonts was listed during the year, though only 

 396 were recorded from the preserved material. The number of 

 species present was doubtless much greater. Those not recorded were 

 found only in the living material and in small numbers. Of those 

 recorded, some forms which might be distinct species were placed 

 together in one because they could not be distinguished during the 

 count. This lack of distinction was sometimes due to the inadequate 

 preservation or to the rarity or to the difficulty of identifying specific 

 characters while counting. Others were placed together because 

 the writer's acquaintance with them was not sufficient for definite 

 recognition. Very many were simply referred to the genus without 

 attempt at species segregation because identification was too difficult 

 to be undertaken in the time available. The question of probability 

 of proper identification will be taken up in the detailed discussion 

 of each form. 



