DESMIDS, DIATOMS, AND TRESH-WATER ALG^. 89 



id 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 to gather them. Usually 

 however, the reader will first see them floating freely 

 about his slide, or attached to various plants. But 

 few are visible to the naked eye except when collected 

 in great masses, and only then as brownish patches; 

 the individual valves are seldom seen without- the 

 microscope, and then only by the most acute and best 

 educated eye. 



They are difficult to study and "to identify. To 

 examine them properly demands the highest-power 

 objectives of the best construction, and a skill in the 

 use of the microscope and accessary optical apparatus 

 not often at the beginner's command. Much has 

 been written about them, but the literature of the 

 subject is so widely scattered through the scientific 

 magazines that only those who make a special study 

 of the subject can hope to have it in their libraries. 

 But the .beginner need not despair. With ease he can 

 learn to recognize a diatom whenever seen, and to 

 know the names of the commonest forms, and this is 

 all he will care to learn at first. Yet he will find it a 

 satisfaction to be able to say to a friend, "That is a 

 diatom," and to explain its box-like structure. 



The following Key has been made to assist the 

 reader in ascertaining the names of a few of the, com- 

 monest fresh-water forms. It is impossible to include 

 even a tithe of the plants, and the microscopist will 

 surely find many not mentioned in the succeeding list, 

 but from the brownish color, the movements common 

 to so many, and the hard, dotted, lined, or otherwise 

 sculptured valves, he can readily know them to be 



