on board H.M.S. 'Challenger. 3 533 



A few remarks on those organisms which have an interest in re- 

 lation to the deposits taking place at the bottom of the sea is all 

 that will be attempted. Soon after the arrival of the Expedition at 

 home I hope to be able to give a more complete report on surface 

 organisms. 



Diatoms. — In the Southern Ocean, south of Kerguelen, in the Arafura 

 Sea, off the coasts of Japan, New Guinea, North America, and in en- 

 closed bays and river-deltas, in short, wherever the specific gravity of 

 the sea is low from an admixture of fresh water, we have met with very 

 many Diatoms on the surface. Taking the maps which Mr. Buchanan 

 has prepared, showing areas of low specific gravity, and comparing these 

 with the records of the occurrence of Diatoms in great abundance, it is 

 significant how often these agree. Again, excepting the area of the 

 Diatom-ooze, those places where Diatoms are abundant, and where the 

 specific gravity of the water is low, coincide generally with the areas 

 where the blue muds are found, i. e. along the continents and great 

 islands. It may be stated generally that close to these shores we have 

 in the surface-waters organisms more or less distinct from those occurring 

 in the open sea. Eesides many forms of Diatoms, the true Noctiluca, 

 several Peridinia, larva) of Annelids, and Mollusca, Hydroid Medusae are 

 more or less characteristic of these shore waters. In the Antarctic, in 

 the Arafura Sea, at Hongkong, off Japan, and elsewhere the tow-nets 

 were frequently filled with a yellow slime in a short time, which con- 

 sisted chiefly of Diatoms. In the Southern Ocean, as has been stated, 

 the Diatoms form a peculiar deposit. In other deposits they are at 

 times also well preserved, but occasionally are either completely masked 

 by shore-debris or removed from the bottom. 



In the open ocean, in the region marked out by the north edge of the 

 N.E. trade-wind and the south of the S.E. trade, small forms of Diatoms 

 do not occur in any numbers. They are represented, however, by three 

 large species, which are undescribed so far as is known. Mr. Wild has 

 figured two of these in Plate 21. These organisms have an exceed- 

 ingly thin coating of silica and are true Diatoms, though they have been 

 confounded with Noctiluca. The nuclei of both these forms are highly 

 phosphorescent, and to them the diffused phosphorescence of the ocean 

 within the tropics is generally due. A third large Diatom has a cylin- 

 drical shape and a much thicker coat of silica, and is not phosphorescent. 

 These three forms are highly characteristic of the presence of tropical 

 oceanic waters. The provisional generic name of Pyrocystis is proposed 

 for the organisms figured in this Plate. 



Coccospheres and Rhabdospheres. — These minute organisms, which are 

 regarded as a]gse, have been found all over the ocean, except south of the 

 latitude of Kerguelen and in some of the enclosed seas. Their remains, 

 in the forms of Coccoliths and Rhabdoliths, make up a very considerable 

 portion of the bottom in some of the Glohigerina-oozes. They are always 



