INTRODUCTION. 



THE Diatomacese comprise an order of microscopic Alga* re- 

 markable for their silicious epiderm and their singular beauty. 

 The silex of their epidermal covering renders all their forms. 

 excepting a few which affect brackish waters, indestructible by 

 the ordinary agency of decomposition. They belong to both 

 salt and fresh waters, though the denizens of the one are not to 

 be found in the other. The marine forms abound in the sea 

 depths, in marshes which are flooded at high tide, in shallow 

 inlets and the muddy bottoms underlying the sandy surface of 

 the seashore. The fresh water forms are plentiful on the mossy 

 stones of mountain streamlets, [tools bordering rivulets, drip- 

 ping-rocks, in ponds, creeks and rivers; every sluggish stream 

 has a bed of them of more or less thickness. 



Owing to their indestructible covering, vast quantities of 

 Diatoms luive formed fossiliferous deposits in many States of 

 the Union, notably Virginia. The city of Richmond is built 

 upon such a deposit from twenty to eighty feet thick and several 

 miles in extent; the Church Hill tunnel was cut through it 

 three-quarters of a mile. 



The silicious epiderm although characteristic of the Diato 

 mace;e. is not without exceptions; in a few it is wholly wanting; 

 others develop it irregularly from almost nothing to nearly per- 

 fect silicih'cation. 



All living Diatoms possess a gelatinous envelope, which, 

 owing to its transparency, can be readily detected only by 

 means of coloring matter added to the surrounding lluid ; it is 

 secreted by the Diatom and is necessary to its existence ; some- 

 times it forms tubes or stalks or stipes which have misled ob- 

 servers into basing genera upon them; nevertheless these are 

 features which should not be ignored. 



The end of these tubes, stalks or stipes attach themselves to 

 stones, wood, and other adjacent objects to prevent the Diatom 

 from being swept away by currents and waves. When the Dia- 

 tom is about to propagate, the frustule is often immersed in 

 large masses of this transparent gelatinous envelope. 



