374 University of California Publications in Zoology [Vol.22 



Variations in Kinds of Organisms 



It might be expected that with such clearly marked periodicity of 

 production, especially as shown by diatoms at the La Jolla pier (fig. 1 

 and table 1), there would be prominent shifts in the specific characters 

 of the diatom and dinoflagellate populations. But for this year at 

 least such distinctions do not appear. 



At the La Jolla pier there were eighteen weeks in 1920 in which 

 production of diatoms might be said to be conspicuous (although not 

 always heavy) and in these eighteen weeks only eight forms were for 

 any week included in the three most numerous. One of these was 

 the catchall assemblage called Chaetoceras sp. which was necessarily 

 indefinite in application. Of the seven species segregated Bacteri- 

 astrum various Lauder was second for two successive weeks; Chaeto- 

 ceras a/fine Lauder was second for one week and third for four 

 weeks; Chaetoceras curvisetum CI. (including Ch. debile CI.) was 

 first for three weeks, second for two, and third for three weeks; 

 Chaetoceras scolopcndra CI. was second once and third once; Lepto- 

 cylindrus danieus CI. was first for two successive weeks; Nitzschia 

 seriata CI. was third in nine weeks, second in four, and third in two 

 weeks; and Skeletonema costatum Grev. was second in three and 

 third in two weeks. 



Omitting the indefinite group Chaetoceras sp., there were only 

 three forms in conspicuous abundance (averaging more than 50,000 

 per liter for one or more weeks) at the La Jolla pier in 1920, viz., 

 Nitzschia seriata CI., Chaetoceras curvisetum CI., and Leptocylindrus 

 danieus CI. The prominence of the last named was confined to the 

 last two weeks in May. One or both of the other two species was 

 conspicuous at all seasons. Such evidence does not indicate any great 

 tendency of the diatom population to change in character from one 

 season to another. 



At Pt. Hueneme the evidence of such change is a little stronger. 

 Six forms of the eight which appeared amongst the first three in one 

 or more productive weeks were the same as at La Jolla. Bacteri- 

 astrum varians Lauder and Leptocylindrus danieus CI. did not 

 reach prominence, but Thalassiosira baltica Grun and Eucampia 

 zoodiacus Ehr. did. The former was very prominent for two weeks 

 at the end of April and beginning of May, and the latter was most 

 prominent during the second week in July. Aside from Chaetoceras 



