460 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



healthy CoscinodiscvH were found in each sample exammed. The whole 

 culture was crowded with Chilomonas in a very active state, which gave 

 the whole contents of the flask a deep red-brown colour. Up to 

 August 24th, the green alga {Plcurococxus) had not become sufficiently 

 abundant to be detected by the naked-eye appearance of the flask, 

 though it could be seen in samples examined with the microscope. 



In another experiment, a flask of Miquel sea-water was inoculated 

 (May 4th, 1908) from two cultures, one containing the green alga 

 {Pleurococcus mucosus) and the other Tlndfissiosira decipicns. At first 

 both did well, and on May 20th (16 days) tliere was a very good crop 

 both of the diatom and the alga. Gradually, however, the alga became 

 predominant, and on October 14th (163 days) only quite empty 

 frustules of Thalassiosira could be found, whilst the growth of 

 Pleurococcus was abundant and healthy. The only case where a diatom 

 was observed to flourish in the presence of this green alga was in 

 a culture of Nitzscliia, a bottom form. In this case a very abundant 

 growth of the diatom was obtained, but tlie Pleurococcnx did not 

 multiply to any extent, although it could always be found on microscopic 

 examination. 



III. NOTES ON rARTICULAII SPECIES OF DIATOMS, 



ON THEIE METHODS OF EEPEODUCTION, AND 



ON OTHER ALO^ OCCUERINO IN CULTUEES. 



A list has been already given (p. 425) of tliose species of diatoms 

 which we have obtained in " persistent " cultures. Of these, a species 

 belonging to the genus Thcdassiosira has been used for experimental 

 work in the great majority of cases. We are not (juite certain as to 

 the identity of the species, but since it most resembles T. dcci^nois, 

 Grun., we have called it by that name, although it does not exactly 

 conform to the published descriptions of that form. The most 

 characteristic feature of this particular species is the eccentric mark- 

 ings on the valves, which are also seen on the valves of the diatom 

 Coscinodiscus exccntricus, Ehr., and, as is typical of the genus, the 

 frustules are united into chains by a delicate filament. Jorgensen (50, 

 p. 96) describes the valves as " decidedly convex," Gran (49) as " flat," 

 and both agree that there are marginal spines and a single asymmetrical 

 spine. Our cultural forms are united together by a filament into 

 chains, some of which are made up of five hundred cells and more, but 

 the distance between each is considerably smaller than that figured by 

 Gran. The valves are quite flat and the marginal spines are often 



