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lowing directions for collecting, preserving, and transporting them 

 have been drawn up in as simple a manner as possible, and in 

 language as plain as was consistent with the information it was 

 desired to impart. 



The directions given should be closely followed, as the methods 

 described have been found, after considerable trial, to be those 

 yielding the most satisfactory results. The Diatomacea) (popu- 

 larly called Diatoms) having been, up to a late date, included 

 among the so-called Infusoria, any directions that have been 

 published for gathering and forwarding specimens of these organ- 

 isms apply equally to the objects under consideration. 



Diatoms are found in a fossil condition as well as in a recent 

 state ; existing forms appearing to differ very little in character 

 from those found in strata below the drift, and presumed to be- 

 long to the miocene and cretaceous periods. As fossil deposits 

 are most readily recognized, gathered, and forwarded, they will be 

 first described. 



Fossil deposits. — These contain both marine and fresh-water 

 species though never in a mixed state; particular strata appear- 

 ing to have been deposited at the bottoms of seas, whilst others 

 have formed beneath fresh water in lakes, bays, or rivers. In 

 some cases the particular species contained indicate the character 

 of the piece of water in which they have grown, different groups 

 of forms appearing in ponds, bays, springs, brooks, and rivers, 

 and at various points of elevation above the level of the sea. 



The principal fossil deposits of Diatoms hitherto discovered 

 contain marine species, and extend over considerable extent of 

 surface. The most important stratum of this character, belonging 

 to the miocene period, has been discovered on the Atlantic coast 

 of the North American continent, extending from the Patuxent 

 river, in Maryland, as far south as the city of Petersburg, in Vir- 

 ginia. How much beyond these two points it extends has not 

 been ascertained, but it is found underlying the cities of Peters- 

 burg, Richmond, and Fredericksburg, in Virginia, and at many 

 other points in that state as well as in Maryland, as at Bermuda 

 Hundred, on the James river, many points on the Rappahannock, 

 and at Nottingham, Piscataway, and Lower Marlboro. It is 

 desirable to obtain specimens from different points in this bed. as 

 it varies in character with every few miles of extent. 



These fossil strata, or as they are commonly called " rufusorial 



