INTRODUCTION. 



serve by the beauty of their forms to dehght the inicroscopist, 

 and by the property which their presence imparts to the Guano, 

 to contribute to the fertihty of our pastures and the growth of 

 our cereals. It is well known that the latter plants contain a 

 large amount of silica in the structures of their stem and leaves ; 

 it is therefore probable that the value of this manure may in 

 some degree depend upon the presence of these minute organic 

 remains, which may thus confer upon the Guano a quality ren- 

 dering it eminently conducive to the healthy growth of such 

 crops. 



When occurring in strata of a fossil or subfossil character, as 

 the deposits of Dolgelly, Richmond, and Lough Mourne, the 

 epiderms of the Diatomacese appear as a white or cream-co- 

 loured powder ; but their living masses present themselves as 

 coloured fringes attached to larger plants — or forming a cover- 

 ing to stones or rocks in cushion-like tufts — or spread over 

 their surface as dehcate velvet — or depositing themselves as a 

 filmy stratum on the mud — or intermixed with the scum of 

 living or decayed vegetation floating on the surface of the water. 

 Their colour is usually a yellowish brown of a greater or less 

 intensity, varying from a light chestnut in individual specimens, 

 to a shade almost approaching black in the aggregated masses. 

 Their presence may often be detected without the aid of a mi- 

 croscope, by the absence, in many species, of the fibrous tena- 

 city which distinguishes other plants : when removed from their 

 natural position, they become distributed through the water, 

 and are held in suspension by it, only subsiding after some httle 

 time has elapsed. 



The frustules of the Diatomaceae, as the individual organisms 

 have not inaptly been denominated, are either free, adherent, or 

 variously aggregated : in Nitzschia, Navicula and others, we have 

 the frustules absolutely free ; in Epitliemia, Cocconeis, &c., they 

 are usually adherent. One mode of aggregation is that of a 

 ribbon-like filament of indefinite length, as in Fragillaria, &c. ; 

 another, that of a zigzag chain, in which the frustules cohere 

 only by their angles, as in Grammatophora, Liatoma and others. 

 In some species the Diatom is provided with a gelatinous 

 pedicel or stipes by which it is united with other frustules, and 



