VIU PREFACE. 



Ehrenberffii, Supp. Plate XXXI. fig. 256, and on that occasion 

 an admirable photograph of the object left me no room for hesi- 

 tation as to the species intended. To avoid confusion and un- 

 certainty in the synonyms, I have also abstained from recording 

 written descriptions, and the references in every case, not other- 

 wise stated, are to the numbers of the plate and figm^e in the 

 work quoted; and, to preserve uniformity, I have invariably 

 used letters for the plate, and numerals for the figure. 



It will be seen that I have omitted all mention of the genus 

 Didyoclia, whose forms, familiar to the English collector, have 

 hitherto been classed vdth the objects described in the present 

 work. This organism has, however, little in common with the 

 true Diatomaceae, and the position and structure of its siliceous 

 skeleton altogether exclude it from the class. 



I have been obliged to defer, until the completion of the plates, 

 all explanation respecting the important function of Reproduction 

 in the Diatomaceae, and a detailed examination of the contro- 

 versy respecting the animal or vegetable nature of these or- 

 ganisms ; neither of these subjects could have been fully discussed 

 without a reference to many of the figures to be given in my 

 second volume, and I have thought it better to postpone these 

 topics, however immediately interesting, than to enter upon 

 explanations that must necessarily have been imperfect or unin- 

 telligible. 



