426 University of California Publications in Zoology [Vol. 22 



several species comprising the group were present in approximately 

 the same proportions in the second year as in the first. 



The Podoplea were present in largest numbers during three months 

 of the first year, September, 1916, and July and August of 1917. They 

 reached their minimum in January. During the second year their 

 occurrence was much more irregular. There was a succession of 

 larger and smaller numbers, the month of greatest abundance being 

 July, followed by March, December, and May. The Podoplea were 

 least numerous in November, with October and January next in order. 

 It is impossible to say what the differences between the two years 



Mean Weekly Abundance of Two Species of Acaetia During the Season of 

 Greatest Abundance. 



The results of the two methods of handling the data are combined. The figures 

 give the average number of animals per haul. 



Clausi Tonsa 



1917 191S 1917 1918 



March 1-7 557 408 July 1-7 7 306 



March 8-14 835 635 July 8-14 37 226 



March 15-21 940 1456 July 15-21 367 5821 



March 22-28 127 1705 July 22-28 53 909 



March 29-April 4 192 461 July 29-August 4 53 275 



April 5-11 32 166 August 5-11 277 754 



April 12-18 . 487 2271 August 12-18 88 242 



April 19-25 . 346 348 August 19-25 343 226 



April26-May2 90 260 August 26-September 1. 154 75 



may mean. The suggestion is not out of place, hoewver, that the 

 relative numbers of the forms making up the mixed group were so 

 dissimilar in the two years that the mean monthly abundance must 

 have been different. This suggestion may be verified from the hauls 

 themselves. 



In order to compare the two years in another way. the mean 

 weekly abundance of the two species of Acartia was determined for 

 the season when the animals were most abundant. A. tonsa was most 

 abundant in July and August and clausi during March and April. 

 The results appear in table 4. A clausi was present in largest numbers 

 during the third week of March. 1917 (March 15-21), and during the 

 week beginning April 12, 1918. But it should be noted that in the two 

 weeks beginning March 15, 1918, there was a marked rise, which was 

 followed by a drop in numbers. After that there came the rise to the 

 maximum. Tn a general way the conditions of occurrence during 

 1917 were similar. Although the time of the actual maximum was 



