1920] Allen: Plankton of the San Joaquin River 25 



figured and described in the literature immediately available for use. 

 Seventh, much of the accurate identification of plankton fonns would 

 require long time and careful work, even for specialists in the various 

 groups. The writer was hampered by lack of time and experience 

 in identification in all groups. Eighth, the synonymy is confusing. 

 This, however, is rather an aggravation than a difficulty in the 

 sense of the foregoing. 



This list certainly seems imposing as stated. As against it, the 

 following facts should be noted. First, that more than one year was 

 spent in studying the living materials, witli both 16 millimeter and 4 

 millimeter objectives, before there was any attempt to count. In 

 this way, sufficient familiarity was obtained with many forms to 

 enable identification even in much contracted, distoi'ted and broken 

 conditions. Second, that most of the names as finally applied meant 

 something definite to the writer, even though there might be error in 

 their specific application. While this fact is unfortunate for the 

 specialist who may wish to know exactly what species are present, 

 it surely leaves the possibility of drawing some valuable conclusions 

 as to seasonal changes, plankton rhythms, and relative numbers. 

 Third, there were enough prominent planktonts, easy to identify, to 

 make a good foundation for a report on plankton characteristics of 

 this region at such stations as were selected. Fourth, those plank- 

 tonts hardest to identify were mostly of the kind which would be 

 largely lost through the meshes of the net, or which were adventitious 

 and so of minor importance in solution of the greater problems of 

 plankton production and distribution. Fifth, the various totals are 

 not much affected by specific errors of identification. 



Estimation of Silt 



In estimating the percentage of silt, the same po.ssibility of error 

 was noticed as that mentioned by Kofoid (1908), i.e., some of the 

 material, being flocculent in character, would appear unduly prom- 

 inent in the Rafter cell ; whereas, the compression of the centrifuge 

 would inake it relatively small in the volumetric record. While it 

 seems probable that difi'erences in the stations and in sea.sonal condi- 

 , tions give this error some real importance, there appears to be no way 

 of avoiding it. There may be some compensation in the fact that 

 with larger quantities of sediment there is usually a larger propor- 

 tion of heavy materials, thus making the compressible materials less 

 conspicuous. 



