2*72 THE AMERICAN MONTHLY [Sept. 



never seen described or figured ; it is as large as 0. im- 

 perator and is apparently divided into separate cells 

 somewhat resembling the filamentous desmid Hyalotheca 

 dissiliens. These cells vary in size, giving the filament 

 an irregular outline ; it is colorless or hyaline, with 

 small and irregular opake patches here and there ; these 

 spots make the revolving motion perfectly obvious to 

 any observer, and its large size and rapid motions make 

 it a remarkable object for observation. I have never 

 found it in masses, but always in separate filaments, 

 traveling over the surface of the mud at the bottom of 

 several feet of water, in company with Pleurosigma and 

 other diatoms. 



The reproduction of desmids and oscillaria, like that 

 of other algse, is conceded to be by means of sexual con- 

 jugation and formation of a sporangium, whose con- 

 tents afterwards separate into spores forming the start- 

 ing point of a new generation. The diatoms also form 

 a sporangial frustule whose valves, in all cases that I 

 have observed, are not only much larger than the parent 

 valves but are different in structure, and the upper and 

 lower valves are unlike ; in this respect resembling the 

 Achnanthes. I have no doubt that the contents of these 

 frustules form spores in a similar manner ; many facts 

 that I have observed would seem to prove this beyond 

 question, but this not the place for an extended argu- 

 ment on this subject. 



The pond holes in the salt marshes are of two classes, 

 the first has a soft bottom of fine grained mud which ex- 

 tends down to the ancient deposit below ; these jjond 

 holes are invariably rich in diatoms and frequently con- 

 tain species that cannot be found elsewhere, and some- 

 times entirely difl'erent from those of neighboring wa- 

 ters. These pond holes are the remains of open water 

 which once covered the site of these marshes ; in those 

 that are quite salt the mud is usually intensely black, 



