26 Univerfsity of California Puhlications in Zoology [Vol. 13 



There is a rather sharp separation of the months named in table 3 

 into two groups according to the prevailing- surface temperatures. 

 This is shown to some extent in table 4, which follows. 













TABLE 



4 













Number of 



Surface 



Hauls 



BY Months and 



According 



TO 



Different 



Temperatures 





Temperaf 



ure 



June 



July 



August 



Sept. 



Oct. 



Nov. 



Dec. 



Feb. 



March 



Api-i! 



14° and 



less 































8 







3 



14?1-15° 





























5 



8 



22 



5 



15?1-16° 

























1 



3 







4 



2 



16?1-17° 





27 



3 







1 







1 



9 







1 



3 



17?1-18° 





27 



1 















2 



















18?1-19° 





11 



9 







3 



2 



1 



















19?l-20° 





2 



12 



14 



1 



12 























20?1 and 



more 







11 



24 



1 



2 























It will be seen from table 4 that during June, July, August, Sep- 

 tember and October the temperature of the surface water was always 

 above 16° C, while during the other months the temperatures were 

 15° C or less. Since there is such a difference in temperatures the 

 first five months in table 4 (spoken of hereafter as "summer months." 

 for convenience) are regarded as a group to be set off against the 

 others, which are called the "winter months." It should be explained 

 that the temperatures were not taken for all the surface hauls, so that 

 there are fewer entered in table 4 than in table 3. August is the 

 warmest month, on the whole, for all hauls were made in water above 

 19°, and Feburary is the coldest, since the temperatures at the time 

 of hauling were all at 15° or less. It may be well to state here that 

 very recently (August, 1913), it has been found that in the vicinity 

 of Santa Monica the water is considerably colder in August than 

 about San Diego, and that Pleurohracliia occurs in large numbers in 

 the cold water. These hauls, however, do not enter into the tables 

 discussed here, for the hydrographic conditions in the more northern 

 locality seem to be so different that the collections made there will be 

 considered by themselves. 



One noticeable thing about the specimens of Pleurobracliia ob- 

 tained in warm surface water, as compared with those taken in colder 

 water, is that all of the former are of small size, while among the latter 



