62 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 3 



For convenience in making comparisons, the same geographical divi- 

 sion of the Gulf into three sections has been used as in the discussion 

 of the 1936 series, namely, the southern section, south of 25°N. lati- 

 tude; the middle section, between 25°N. and 27°N. latitude; and the 

 northern section, north of 27°N. latitude. 



Toward the preparation of this report Professor Allen contributed 

 the microscopic examination and Dr. Cupp contributed the general 

 discussion, the taxonomic discussion, and the illustrations of prominent 

 species. 



Topography of the Gulf 



Certain features of the topography of the Gulf were discussed in 

 the paper by the junior author dealing with the 1936 series. Only a 

 brief review of the salient points need be repeated here. The northern 

 section, according to available information, appears to be a more or 

 less steeply descending trough extending from the comparatively level 

 floor (250 meters deep or less) above Angel de la Guardia Island to 

 27 °N. latitude. Here the bottom is uneven and at least part of the 

 trough seems to terminate. There is little evidence of land drainage 

 from the side of the trough and probably there is little direct influence 

 exerted by ocean waters on the productivity of this section. The Colo- 

 rado River drainage should have considerable influence. The trough is 

 partly filled by Angel de la Guardia, Tiburon, and other islands. The 

 remainder of the Gulf, south of 27 °N., is characterized by a broad, 

 open, south tilted basin. Ocean waters would naturally be expected 

 to have considerable influence upon productivity in the middle section 

 of the Gulf, and to be of major importance in the southern section. 



Numerical Abundance of Diatoms 



Far larger numbers of diatoms were present in the samples collected 

 during the present Expedition than in the 1921 or 1936 series. Sixteen 

 of the sixty-four catches (25%) in the Gulf had over 100,000 cells 

 per liter, ten of these over 1,000,000 cells. The largest catch was 

 recorded as having over 5,000,000 cells per liter. In 1921, nineteen 

 catches out of a total of 117 (16-f-%) had over 100,000 cells per liter 

 but none reached 1,000,000 cells. The largest abundance recorded in 

 the series was 592,720 cells per liter. In the 1936 Hancock Expedition 



