PREFACE. 



cally unworked. Under each species we give its dis- 

 tribution in tlie British Islands so far as it is known, 

 and this will at once indicate the extensive and repre- 

 sentative area which has been examined. 



We also give the known geographical distribution 

 of each species. It must be remembered, however, 

 that our knowledge of the geographical distribution of 

 Desmids is at present very incomplete. 



.Most of the figures are original, and much time and 

 care has been expended on the drawing of the speci- 

 mens in order that they should be as accurate as 

 possible. In many instances we have given an accu- 

 rate copy of the original figure published by the author 

 of the species, but in all cases where the figures are 

 not our own, mention has been made of the fact. In 

 some cases where we have not had a drawing of a 

 British specimen in a convenient form for reproduc- 

 tion, we have not hesitated to give a figure of a foreign 

 specimen, provided the latter was typical. We regret 

 verv much that the figures are not drawn to a uniform 



t/ 



scale of magnification, but the great diversity of size, 

 and of firmness and structure of cell-wall exhibited 

 throughout the family, together with the fact that we 

 have reproduced a large number of the original figures 

 of various authors, has rendered such uniformity of 

 magnification impossible. Moreover, many species vary 

 very considerably in their dimensions, so that relative 

 size is of i/<> fiunJci mnthil iiiij>ortitct' as a specific dis- 

 tinction. Hence, it does not matter what the magnifi- 

 cation of the drawing is, provided the scale is large 

 enouo-h to show all the distinctive features of the 



o 



species. We think, however, that the student will 

 soon become acquainted with the relative sizes of these 

 plants, and we have endeavoured to make the measure- 



