NO. 4 ALLEN : PLANKTON DIATOMS 51 



miles from Georges Bay at the northeast end of the Gulf. As far as 

 series taken at different seasons and in different years may be regard- 

 ed as comparable, the 1921 series supports the 1936 evidence that the 

 upper third of the Gulf is a productive region for plankton diatoms. 

 In addition, it points to a strong influence of the Colorado River in 

 favor of productivity. No other collections from this section are on 

 record. 



Middle Section 



Twelve stations were occupied in the middle section. From them 

 eighteen catches were taken. Only four of these were so small as to be 

 negligible. Since they were taken more than two weeks earlier than 

 the other fourteen, it is not certain that their Insignificance indicated 

 real differences. In fact, the probability is to the contrary, because 

 later catches of larger size were taken so nearly at the same locations 

 that they suggest a time difference rather than a locality difference. 



However, no catch was really large. Of the seven showing moder- 

 ate size (10,000 cells or more per liter), one was In Concepclon Bay 

 and five at two stations in mid-Gulf a little north of the latitude of 

 Topolobampo Bay. The remaining catches, showing only thousands 

 of cells per liter, were all near the western shore of the Gulf, two In 

 Concepcion Bay. The one fairly large catch In Concepclon Bay 

 suggests favorable influence of land and shallow water, but the five 

 in mid-Gulf were larger than that, and two others nearly equal. 

 Apparently, the favorable condition in mid-Gulf may be due to ocean 

 Influence to an appreciable extent. At any rate, it is true that these 

 better catches of moderate size were made south of the low cross- 

 ridge which Interrupts the continuity of outward slope of the floor of 

 the Gulf, and it was almost at the point of beginning of the part of 

 the Gulf which appears to slope continuously to the ocean. In 1921, 

 some large catches were made by the expedition of the California 

 Academy of Sciences In this section in April and June, notably near 

 Carmen Island (Allen, 1923). In November, 1935, only four months 

 before the 1936 catches were taken in mid-Gulf, the Templeton 

 Crocker Expedition of the California Academy of Sciences obtained 

 a large catch near Topolobampo Bay (Allen, in press). Although ob- 

 tained in different years, the fact that large abundance has been 

 found at three different seasons of the year in this section of the 

 Gulf Indicates that It tends to be productive of ocean pasturage. 



