804 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



bility that it contains organic matter, which renders it nutritious 

 in itself. When thus occurring in strata of a fossil or sub-fossil 

 character, the Diatomaceous deposits are generally distinguisha- 

 ble as white or cream-colored powders of extreme fineness. 



192. For collecting fresh Diatomacece, three general methods 

 are to be had recourse to, which have been already described 

 ( 143, 171). " Their living masses," says Prof. W. Smith, " pre- 



FIG. 102. 



Fossil Diatomacece, &c., from Mourue Mountain, Ireland: a, a, a, Gaillonella (Meloseira) procera, 

 and G. granulata ; d, d, d, G. biseriata (side view); 6,6, Surirella plicata; c, S. craticula; fc, S. cale- 

 donica; e, Gomphonema gracile; /, Cocconema fusidium; g, Tabellaria vulgaris; h, Pinnularia dac- 

 tylus ; i, P. nobilis ; I, Synedra ulna. 



sent themselves as colored fringes attached to larger plants, or 

 forming a covering to stones or rocks in cushion-like tufts or 

 spread over their surface as delicate velvet or depositing them- 

 selves 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 color is usually a yellowish-brown of a greater 

 or less intensity, varying from a light chestnut, in individual spe- 

 cimens, to a shade almost approaching black in the aggregated 

 masses. Their presence may often be detected without the aid 

 of a microscope, by the absence, in many species, of the fibrous 

 tenacity 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 little time has elapsed." Notwithstanding every care, the 

 collected specimens are liable to be mixed with much foreign 

 matter ; this may be partly got rid of by repeated washings in 

 pure water, and, by taking advantage, at the same time, of the 

 different specific gravities of the Diatoms and of the intermixed 



