INTRODUCTION. IX 



In a postscript to the letter announcing the above discovery, 

 Mr. Thwaites states that he had subsequently detected the 

 process of conjugation and mature sporangia in two species of 

 Gomphonema and in Cocconema lanceolatum ; and in a paper 

 which appeared in the ' Annals ' for November 1847, he details 

 and figures these and other cases of conjugation, in the dis- 

 covery of which I had the pleasure of aiding and co-operating 

 with my acute friend. 



In a final communication which appeared in the ' Annals ' for 

 March 1848, Mr. Thwaites records two additional examples of 

 the conjugating process in Cyclotella Kiltzingiana and Schizo- 

 nema {Colletonemd) subco/iarens, and shows that the enlarged 

 frustules of the Melosirea, which Kutzing had conjecturally 

 regarded as reproductive bodies, were in fact the sporangial 

 product of conjugation, the original sporangia submitting to 

 self-division immediately on their formation, and thus forming a 

 chain of frustules larger than those from which they had them- 

 selves originated (Plate XLIX. fig. 329). 



No further observations on this singular process appear to 

 have been recorded by British naturalists until the appearance 

 of the first volume of the present work, in which I incidentally 

 mentioned the names of various species in which I had detected 

 conjugation, deferring a detailed account of the phsenomena 

 attending the process until the present opportunity of recording, 

 in consecutive order, all the examples I had noticed in the 

 British genera. 



In the 'Annals of Natural History' for August 1855, an 

 interesting addition was made to the number of genera exhibiting 

 conjugation by Dr. J. W. Griffith, who had observed the process 

 in a Navicula (probably N.jirmd) occurring to him in a ditch 

 near Blackwall. The ' Micrographic Dictionary,' in part edited 

 by the same author, in plate vi. fig. 5, March 1855, gives a repre- 

 sentation of Suriretta bifrons, Kiitz., in a state of conjugation : 

 and lastly, in the 'Annals of Natural History,' January 1856, 

 we have an account of the phsenomena attending the process in 

 Cocconeis Pediculus, Cymbella Pediculus, Kiitz., and Amphora 

 ovalis, as noticed by Mr. H. J. Carter of Bombay, all of which 

 appear to have occurred to him in vessels containing larger 



