INTRODUCTION. 7 



Our knowledge of the life-history of the algae must make very many advances 

 before the true system can be developed, and abstinence from adding to the present 

 numerous classifications is an exhibition of self-control not very common. 



There are, however, certain great groups, which are already plainly foreshadowed, 

 and which no doubt will be prominent points in the perfected classification. 

 Amongst these are the Conjiigatce, or those plants in which sexual reproduction 

 occurs by the union of two similar cells. In the present paper all the plants of 

 this family described are together, since the diatoms are not noticed ; but in Raben- 

 horst's work the latter plants are very widely separated from their fellows, and this 

 seems to me the weak point of the Professor's system. 



