DESMIDS AND DIATOMS. 509 
reproduction of the Diatomacee, and as it is chiefly of the 
latter that this article is designed to treat, it is unneces- 
sary to enter into a more minute description. 
Before dismissing the subject of the Desmidiacee, I may 
say that they are exceedingly common, especially in open 
tracts, abounding in exposed localities. They are ex- 
ceedingly beautiful, and, with the Diatoms, which are 
always associated with them, form objects well worthy of 
search on the part of those provided with the proper 
instruments to discern their beauties. They are very 
tenacious of life, I having frequently found them still 
green and healthy in bottles of water from which all 
other varieties of vegetable life had long since disap- 
peared. As agents in the mechanism of Nature, the Des- 
mids play an important part, not only by directly serving 
as the food of many aquatic animals, but also by their 
furnishing, in company with other plants, the oxygen 
which is so necessary for the purity of the water in which 
these animals are to dwell. 
Lastly, the Desmids have a peculiar interest from the 
fact, that, notwithstanding their being destitute of the 
hard parts which constitute the fossil remains of other 
plants and animals, they are yet found in considerable 
numbers in the fresh-water marls of North America, and 
bodies bearing a striking resemblance to their Sporangia 
occur tbhindantly i in the silicious nodules of the chalk, 
and even in the flinty hornstone of the Silurian and 
Devonian eras.* To this fact allusion will again be made 
in the diseussion of the geological relations of the Diato- 
macez, 
*It is generally stated that these organisms, sign l be eegnen are the spora 
gia of Desmids; ut if so, how is it that they occur in rine deposits, suc ter for the 
chalk and horn while the recent Desmids are exclusi ely fresh-water forms : 
In alusi ions to ae e fossils this fact seems to have been generally overlooke 
