no ANNALS OF SCOTTISH NATURAL HISTORY 



of varieties ! Varieties are founded on slight differences of 

 shape, or size, or of the degree of roughness or smoothness 

 of the particular forms. But it is well known that many 

 species vary greatly in size and roughness. Take the case 

 of Gonatozygon Brebissonii, De Bary. This varies in surface 

 from perfectly smooth without puncta to rough with gran- 

 ules, depending not a little, I believe, on the water in which 

 it is found, and in size from the tiniest thing to 300/-6 long 

 or more ; but it is distinctly G. Brebissonii in all these 

 variations. If there are no intermediate forms, I certainly 

 prefer to call the extremes separate species. 



There is another subject on which a remark may be per- 

 mitted. It is in connection with the attempts — more or less 

 successful, generally less — to divide some of the old genera 

 into new. I confess to having a certain amount of sympathy 

 with this movement, for the genera Staitrastrum and Cos- 

 marium are becoming very unwieldy. But however desir- 

 able a rearrangement would be, it seems to me that the 

 present state of our knowledge does not admit of an arrange- 

 ment on a strictly scientific basis being made. Any other 

 would simply add to the confusion already existing. Before 

 a scientific arrangement is possible, the Desmidieae of the 

 whole world must be much better known. Thanks to the 

 few hard workers in this field, this knowledge is rapidly being 

 gained. 



In this paper I adhere mainly to Ralf's divisions. I 

 decline to break up Docidium into Docidhim and Pletiro- 

 tamium. In dealing with species when the cells are empty, 

 as they usually are in material sent from other parts of the 

 world, how is the chlorophyll to help one? Triploceras is 

 a very natural division ; and I do not agree with Wolle in 

 retaining it under Docidium. 



Collections of material for examination are gratefully 

 acknowledged from the following : Shetland, Mr. John Sim, 

 who also sent many valuable gatherings from Aberdeen and 

 Kincardine ; Orkney, Mr. Cowan ; Caithness, Mr. James 

 Mitchell; Ross (West), Rev. D. Campbell; Ross (East), the 

 late Dr. D. M. Fraser ; Ross (from numerous localities about 

 Strathpeffer, the Black Isle, etc.), Mr. and Mrs. Farquharson 

 of Haughton ; Inverness (Glen Urquhart), Dr. A. M'Gillivray; 



