INTRODUCTION. 



Section I. 

 Habitats and General Appearance of the Diatomace.^. 



The Diatomacese are readily distinguished from the Desmidiege, 

 the Pahnellaceae, and other miiceUular Algae, by the possession of 

 an epidermal covering of silex, which renders their forms inde- 

 structible by the ordinary agents of decomposition. They are 

 all exceedingly minute, and require the human eye to be aided 

 by the various appliances of the optician's art, that thek beauti- 

 ful forms may become objects of admiration and study. They 

 inhabit the sea or fresh water, but the species peculiar to the 

 one are never found in a living state in the other locality, 

 though there are some which prefer a medium of a mixed 

 nature, and are only to be met Avith in water more or less 

 brackish. The latter are often found in great abundance and 

 variety in districts occasionally subject to marine influences, such 

 as marshes in the neighbourhood of the sea, or the deltas of rivers, 

 where, on the occurrence of high tides, the freshness of the water 

 is affected by percolation from the adjoining stream, or more 

 directly by the occasional overflow of its banks. Other favourite 

 habitats of the Diatomacese are stones of mountain streams or 

 waterfalls, and the shallow pools left by the retiring tide at the 

 mouths of our larger rivers. They are not however confined to 

 the localities I have mentioned, — they are in fact almost ubiqui- 

 tous, and there is hardly a roadside ditch, water-trough or 



