DESMIDS, DIATOMS, AND FRESH-WATER ALG^E. 91 



In remote ages diatoms existed in even greater num- 

 bers than at present. Immense beds of fossil frustules 

 are found in many parts of the world, especially in our 

 own country. In Maryland and in New Jersey diato- 

 maceous earth is obtained containing exquisite forms. 

 In Virginia a certain deposit is especially renowned, 

 since it is eighteen feet thick and underlies the city of 

 Richmond. This has afforded the student some of the 

 rarest and most valued frustules, or valves, for the frus- 

 tule, before it can be properly studied, must be sepa- 

 rated into its two valves. To have produced such a 

 mass they must have existed in incalculable numbers in 

 a great body of water where, dying, and sinking to the 

 bottom year after year, their skeletons accumulated as 

 others continued to fall. To appreciate the probable 

 length of time, as well as the number of diatoms, need- 

 ed to make such a deposit, it is only necessary to know 

 that a single frustule is seldom thicker than the one 

 ten-thousandth of an inch. 



At the present day living diatoms are often found in 

 large numbers forming a yellowish-brown film on the 

 mud in shallow water. In such cases it is no trouble to 

 skim them up and so gather them. Usually, however, 

 the beginner will first see them floating freely about his 

 slide, or attached to various plants. But few are visi- 

 ble to the naked eye except when collected in great 

 masses, and only then as brownish patches ; the indi- 

 vidual valves are seldom seen without the microscope, 

 and then only to the most acute and best educated eye. 

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