INTRODUCTION. 3 



wonderful symmetry and their elegance of form. This 

 feeling 1 is highlv intensified on observing the great 



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variety of the forms, and astonishment increases when 

 the beautiful ornamentation of some of them becomes 

 manifest. Diatoms are admitted by all to be very 

 beautiful microscopic objects, but they are far sur- 

 passed in elegance by Desmids. 



The simplest forms are seen in the genera i\Fcxn- 

 l;vninni, Cylindrocystis, Peninni, and Spirotaenia, which 

 consist of cylindrical or subcylindrical cells, usually 

 without any constriction. Even in some of these the 

 cell- wall is ornamented. In some species of Pen-in in- 

 there is a distinct although slight median constriction, 

 which is the first indication of the feature so cha- 

 racteristic of most Desmids, riz. the constriction of the 

 cell into two semicells. In the large genus Closterium, 

 and also in Rot/a, and one or two other small genera, 

 there is no constriction, but in those specified there is 

 more elegance of form, many species of the former 

 genus being most gracefully curved and gradually 

 narrowed towards the extremities, whilst some of them 

 have a slight ornamentation of the cell-wall. This is 



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usually in the form of longitudinal striations. In 

 Doridiwn, which is subcylindrical, there is always a 

 division into two semicells with an ornamentation at 

 the base of each, and some species possess a grace- 

 fully undulated outline. In Pleurotdenium the apices 

 are often ornamented. In Tetmemorus the apices are 

 notched and the cell-wall is generally punctate. In 

 most of the other genera of Desmids there is a 

 more or less deep constriction, the semicells being 

 connected by an isthmus of variable breadth. In 

 Cosmarium, which is by far the largest genus of the 

 family, many of the species exhibit an exquisite orna- 

 mentation of the cell- wall, being decorated with sym- 

 metrical patterns of granules, papillae, or warts of 

 various sizes and shapes. The next genus in point of 

 view of numbers is Staurastrum, and in this genus is 

 found the greatest diversity of form. In addition to 



