12 INTRODUCTION. 



teristic of the family. In this country the conjugation of 

 about forty species has been noticed by different observers, 

 and M. De Brebisson informs me that he has gathered spo- 

 rangia of the following species in France : — Hyalotheca dis- 

 siliens, Didymoprium Grevillii, Staurastrum pygmcBum, S, con- 

 troversum, S.muticum, Cosmarium Botrytis, Closterium Lunula ^ 

 C. setaceum, C. acutum, and C. lineatum. 



In defining the genera and species, I have made no use of 

 the reproductive bodies, for as yet too little is known about 

 them to render them available for that purpose, and in many 

 cases we are still uncertain whether the mature form is yet 

 known. They are likewise so early detached from the 

 emptied cells, that it is often very difficult to determine to 

 what species they belong*. 



I have gathered sporangia of other species besides those 

 mentioned in this work ; but whenever they were not still 

 adherent to the fronds I have thought it best to pass them 

 over, lest I should be the cause of error. 



That the orbicular spinous bodies so frequent in flint are 

 fossil sporangia of Desmidiese cannot, I think, be doubtful 

 when they are compared with figures of recent ones. Indeed 

 one celebrated geologist. Dr. G. Mantell, who, in his ' Medals 

 of Creation,' without any misgiving had adopted Ehrenberg's 

 ideas concerning them, has changed his opinion, and in 

 his last work regards them as having been reproductive 

 bodies, although he is still uncertain whether they are of 

 vegetable origin. 



Ehrenberg and his followers describe these bodies as fossil 



* A principal use of generic and specific characters is to enable us to 

 identify the species we meet with ; and although a genus may be accurately 

 defined by characters taken from the reproductive parts, yet if that definition 

 can be tested only in a few rare instances, it will be inferior in real value to 

 one which is derived from less important parts that are always present. My 

 wish has been to render the present work a practical one, useful in the fields 

 ;us well as in the study : I have therefore omitted the employment of such 

 characters, and endeavoured to express those which I have adopted as con- 

 cisely as may be compatible with usefulness. 



