IV TREFACS. 



former editions_, wliile the introduction of the Tables from Part II. 

 has further extended it ; but, as I have been anxious to give an 

 impartial account of the researches on this subject, a briefer summary- 

 might have impaired its usefulness and value. To Dr. Arlidge is 

 due the rearrangement and preparation of this part. 



Part II. contains descriptions of the Families, Genera, and Species 

 of the groups whose general history forms the subject of the preceding 

 part of this volume. The systematic arrangement of Ehrenberg has 

 been retained for the Phytozoa, Protozoa, and Rotatoria, the new 

 genera and species of other naturalists being collated and engrafted 

 thereon. The descriptions of those curious and higlily-organized 

 creatures the Rotatoria have been extended and revised by Professor 

 Williamson of Manchester, whose original researches and observa- 

 tions on this group are greatly appreciated, both in this country and 

 abroad. 



In consequence of the long illness of Mr. Ralfs, who had under- 

 taken the re\dsion of the Bacillaria, the publication of this edition has 

 been delayed, and that group has been printed last — a deviation from 

 the original design which it is hoped will not inconvenience the 

 reader, while it has allowed opportunity for the insertion of the latest 

 researches. Owing to the circumstance stated above, the revision of 

 the Systematic History of the Family or Subgroup Desmidiacese has 

 been kindly carried out by Mr. William Archer of Dublin, who has 

 added some original views, expressing by symbols the characters of 

 certain genera ; moreover, M. de Brebisson of Falaise has given this 

 edition the benefit of his valuable co-operation, by furnishing descrip- 

 tions of the newly- discovered foreign species. 



The elegance and variety of the forms, the beauty and elaborate 

 sculpturing of the silicious shells of the Diatomacese, and the general 

 interest now taken in their study, rendered it desirable to bring together 

 in this volume ail the known genera and species, British and foreign. 

 This I have been able to effect by the research of Mr. Ralfs, whose 

 name is so intimately identified with the knowledge of these organisms, 

 and whose present arrangement of their families and genera will no 

 doubt tend to facilitate our better acquaintance with them. Owing to 

 the great dimensions which this treatise has acquired, and the limited 

 space consequently at command, I was under the necessity of con- 

 densing the manuscript of Mr. Ralfs, and of introducing abbrevi- 



