306 DESCRIPTION OF [Puli/ffastrica 



In pure river or spring water these little beings are not found ; 

 but on the banks of rivers and ditches, about the source of springs, 

 and also in gutters and moist places generally, they are abundant. 



The fine fibrous Alga), marine and freshwater, are often richly 

 clothed with them, sometimes indeed completely overgrown by them. 

 They are common, too, in summer, in small ponds, and in the mud on 

 the sides of ditches, and, when numerous, impart a brown colour. 

 On warm summer days, they are often raised to the surface by the 

 oxygen gas they generate, and which adheres to them, when they 

 form thin, delicate films on the siu-face, and sometimes a layer of 

 greater thickness, mucous and compact. Otherwise, they occur 

 in aggregated, iatricate masses. The microscope shows the thin 

 pollicle oftentimes made up of various species, mostly Naviculce, 

 Oymlellm, Surirellce, or free Synedra, congregated together, and 

 having a more or less lively motion. The mucous and larger masses, 

 generally have but one prevailing species. 



The 3IehsircB {Gallionella, Ehr.) build brownish, conferva-like 

 masses, which afiix themselves to plants, or stones, or other solid 

 substances in the water; other forms, as Fragilaria, occur gene- 

 rally on decaying wood or leaves, or amongst Conferva, mingled with 

 Cymhellce, Synedra, &c. It seldom happens that one form is found 

 alone. Larger specimens are mostly met with in aggregate, attached 

 masses ; smaller when isolated, free and moving about. 



Brackish water, where a river discharges itself into the sea, or 

 where the sea water rises with the tides and mixes -with the fresh 

 water of the stream, are localities in which Naviculacea abound. 

 Such water, on being passed through a filter, leaves aU its larger 

 particles behind on the cloth, and these can be collected and prepared 

 as may be required. AVhen we have to deal with a film on the 

 surface, it may be removed with a spoon or spatula, and placed on 

 paper ; where the frustules grow on Algse, the two may be removed 

 together. 



In the case of fossil Diatomacea, which mostly occur mixed with 

 various mineral particles, they may be separated for examination, 

 as described in Part II. page 110. 



The following veiy ingenious plan of getting transverse or oblique 

 sectiouB of the smaU lorica: of Xavuuh, and of other Diafomecc, is 



