to exude a mucilaginous substance with such force 

 as to propel itself through the water in the manner 

 of a hydraulic submarine boat. Not only at the 

 present time are these plants giving their quota to 

 fresh-water and marine deposits, they have also in 

 the past contributed quite an abundance. When 

 the diatom dies, its silica shell sinks to the bottom 

 of the water in which it lives, and there forms part 

 of the sediment. The later hardening of this sedi- 

 ment into rock leaves the glassy frustules unal- 

 tered. Indeed, almost pure cultures of fossil dia- 

 toms, so to speak, exist in great tracts throughout 

 the world. Perhaps the most remarkable is that of 

 Richmond, Virginia; it is miles in area, and in 

 many places it is more than forty feet in depth. 

 Yet it is a noteworthy fact that the most ancient 

 fossil diatoms found differ not in slightest essen- 

 tial degree from those living to-day. In truth, in 

 most respects former genera and even species are 

 like their modern representatives; most of them in 

 the minutest detail are identical. 



II 



Our first view of the diatom P. angulatum un- 

 der the lens and enlarged one hundred diameters 



[120] 



