Xhe following paper is the result of my exaiuination of four coUections, viz: 



A series of samples collected in March 1897 by the late Dr. E. Nyman on the 

 route from A den to Java. 



A series of samples collected in Februarj^ and March 1899 on the same route by 

 the late Dr. C. Aurivillius. 



A considerable number of samples collected, also by Dr. C. Aurivillius, from May to 

 September 1899, in the Malay Archipelago, from Billiton to Tiraor. 



An extensive series collected by Dr. R. Willemsen of the Dutch Navy on the 

 route from 45° S. 22° E. (end of December 1899) to 30° S. 91° E. (middle of January 

 1900) and from the last point to 2° N. 94° E. (beginning of February 1900). 



The samples procured by Dr. Nyman were all small and had been collected by 

 means of pumping water through a fiiie silk-net. No observations on the tempei^ature 

 or the salinity of the water correspond to these samples, which were examined for diatoras, 

 dinoflagellates and tintinnodea, but not for other organisms. 



The series obtained by Aurivillius from Aden to Java were collected by filtering 

 water drawn from the sea. Observations on the temperature were carried out and samples 

 for determination of the salinity were taken simultaneously. 



The very rich collection from the Malay Archipelago had been obtained by means 

 of a tow-net. This collection belongs to tlie »Riksmuseum» of Stockholm, and I take 

 this opportunity of thanking Prof. Hj. Théel, who delivered the collection to me for 

 examination. ' ■•' '■ ' '■' -■ ■ ' ''i ' - '- i -- ;■ 



The collection of Dr. R. Willemsen on board the steamer »Tromp» of the Dutch 

 Navy, contained 37 samples, all obtained by pumping water through a silk-net. Obser- 

 vations on the temperature of the water were carried out at the same time as the samples 

 of plankton were taken, and, besides, water for chemical tests was bottled. For this very 

 valuable collection 1 beg to offer Dr. Willemsen my best thanks. 



The determinations of the salinity of the water have been cariied out by Professor 

 S. O. Pettersson, who kindly procured me all hydrographical data. 



Although the collections contained a very large number of species, they cannot 

 aflbrd a complete insight into the plankton of the Indian Ocean. Väste regions remain 

 at present unexplored and, besides, no samples have been collected in the Ocean from 

 April to December. Therefore I consider the following data merely as rudimentary ma- 

 terial for future generalisations. 



In the following account I use the signs t. for the temperature of the water in 

 centigrades, s. for the salinity pro mille; rr. denotes very rare, r. rare, -|- not rare, 

 c. coinmon, cc. very common. 



