208 DISCOVERY REPORTS 



With generic names there is some difference. It will always be a matter of opinion 

 how many genera are necessary to interpret adequately any given population, but pro- 

 viding such names are legal and valid the number is of little consequence. It is sur- 

 prising, however, that a large number of generic names now in use are illegitimate, and 

 have no legal standing, or are used in an entirely different sense from that in which they 

 were created. Pleurosigma, Rhabdonema, Nitzschia, Rhizosolenia, and in a different way 

 Melosira, are a few examples included in this work. In view of the fact that the Inter- 

 national Botanical Congress has taken steps to enquire into the position of the generic 

 names of diatoms, with a view to recommending for conservation illegitimate names in 

 current use, these names have been provisionally accepted in the sense in which they 

 are used to-day. 



TAXONOMIC NOTES 



The following notes seek to explain the more difficult problems that have arisen in the 

 preparation of this systematic account, and to give a general outline of the methods of 

 dealing with the more complicated groups. The treatment of some of the larger genera 

 is quite orthodox despite the fact that revision in some cases is urgently needed. When 

 dealing with such groups it has been considered advisable to recognize as many divisions 

 as are deemed necessary for the interpretation of the diatom population under con- 

 sideration. 



Throughout this work every effort has been made to regard all of the problems from 

 a biological point of view and to seek a connection between the terms, species, structure, 

 function and environment. 



For convenience of reference the notes are arranged under subordinal headings in 

 systematic sequence. 



Discineae. 



The family Coscinodiscaceae claims by far the greatest variety of genera found in the 

 plankton, representing about 30 per cent of the total number of species. 



In the genus Melosira four species only were observed, Melosira sol being fairly 

 abundant at St. WS 481. The peculiar form Melosira sphaerica Karsten was encoun- 

 tered occasionally but never in great numbers. This species I regard as a truly plank- 

 tonic, cold-water form which prefers a low salinity ; I doubt very much whether it is a 

 true Melosira, but could not obtain it in sufficient quantities to make as detailed an 

 examination as I would have wished. The plasticity of its cells and its irregular chain 

 formation indicate special modifications to meet oceanic requirements. 



Hyalodiscus was poorly represented ; three species were recorded when the net touched 

 bottom off South Georgia. Skeletotiema was represented by one species only, namely 

 S. costatum (Greville) Cleve, which occurred frequently in the plankton of the Peru 

 current, taken on a line of inshore stations between 20 and 40 S. 



Two species of Stephanopyxis were recorded, namely, S. Pahneriana and S. turn's, 

 the former being abundant off the east coast of Africa and around the Cape of Good 

 Hope. Great difficulty was experienced at times in separating these two species, for 



