PREFACE. 



WHILE the Preface forms the first part of every work, as 

 the name implies, and precedes always the descriptive matter, 

 it is as invariably the last which is written, and the last often 

 to be read. 



Nevertheless there are few portions of a work the endit- 

 ing of which is undertaken with more alacrity than the 

 writing of this same preface the author's mind being 

 cheered by the near prospect of the completion of his task 

 it may be of a difficult and an arduous task in which the 

 obstacles to its successful issue were many, and the result prob- 

 lematical, and the hope of a suitable reward in recompence 

 of his toil either in the form of approbation, if deserved, or 

 money, should the undertaking be of a sufficiently popular 

 character to lead to so substantial a termination. 



There is also another reason which renders the writing of 

 the preface an agreeable occupation. It is the opportunity 

 which it affords to the writer to render to those who have 

 aided him in his enquiries the acknowledgments which their 

 liberality and their kindness so strongly claim. 



To Sir "W. J. Hooker, Dr. Greville, Mr. Borrer, Mr. 

 Harvey, and the Rev. M. J. Berkeley, names prominently 

 inscribed in the records of the successful cultivators of natural 

 science, the pursuit of which is the most peaceful and the 

 most pleasant of all employments, I am deeply indebted. 

 The two former gentlemen liberally placed at my disposal 

 their valuable collections of freshwater Alga, and Mr. Harvey 

 and Mr. Berkeley communicated to me numerous unique 

 specimens accompanied by valuable remarks. 



