NATURAL HISTORY OF THE DIATOMAEGdE. 457 



currents. Upon these foundations Maury has formed theories which, 

 however true they may be in themselves, are not borne out by later 

 researches, for the forms which Ehrenberg supposed to be peculiar to 

 certain quarters of the globe have been found to be almost universally 

 distributed ; and, therefore, any deductions, which may have been estab- 

 lished in consequence of their appearance upon a ship, do not prove that 

 they were brought from the spot where they were first seen. In fact, 

 the diatomaceae would seem to be more widely distributed than any other 

 group of organisms, animal or vegetable ; and the student of them need 

 never be at a loss for specimens to examine. The pool by the road-side, 

 the mud of the river bank, the moss upon the house-top, the earth be- 

 neath his feet, or the air above his head, may be searched, and will all of 

 them yield him material for observation, wonder, and delight. In the 

 living state, and, as often found, floating upon the surface of the water 

 of a pond or slowly running river, swamp, or ditch, the diatoms present 

 themselves as a flocculent collection of more or less dark rust-colored 

 matter, coherent in stringy masses, when such genera as ]\Iclosira, Frag- 

 ilaria, or Himantidium occur, or consisting of particles readily dispersed 

 and scattered when Navicula, Pinnularia, or other so-called free genera 

 exist. The color of such a mass may vary from a golden orange to a 

 dark brown, according to the thickness of the stratum or particular spe- 

 cies present, or, it may take on a greenish tinge at certain seasons, which 

 is supposed to indicate a change in the character of the endochrome 

 having some connection with the process of reproduction. At times, I 

 have found that bright green masses of floating conferva:, — which are 

 filamentous water-plants found in all waters, both fresh and salt, — will 

 yield beautiful specimens of diatoms, which arc entangled among their 

 branches, or grow adherent to them. But in most of such cases the 

 species belong to the group of adherent forms, and for those we are to 

 look to submerged plants, sticks, metal, and stones, and there they 

 appear as a brownish or fawn-colored mass, either closely adherent, or 

 with its free ends floating freely in the water, as delicate threads, borne 

 hither and thither by the changes of the current. A sprig of some 

 submerged plant, bearing a cluster of some such genus as Himan- 

 tidium, Fragilaria, or Tabellaria, presents a beautiful object, as the fine 

 hair-like filaments spread out on all sides, or bend with the motion of the 

 VOL. i. 60 



