DESCRIPTION OF PLATES. 181 
Diatoms. (Puates XII, XII, XIV., XV. anp XVI.) 
Diatoms are unicellular vegetable organisms 
which abound in fresh, brackish, and salt water, 
where they are usually to be found adhering to 
stones, to various water-plants, or to decaying 
vegetable material. They belong to the Alge, 
and contain a coloring matter which appears to 
be a modification of chlorophyl, but which has 
a yellowish-brown color instead of the bright 
green hue which this pigment has in higher 
plants. This is in minute granules, or is dif- 
fused through the cell contents — endochrome. 
The peculiarity of diatoms which gives them 
a special interest for microscopists, consists in 
the presence of a rigid casing of silex, which 
encloses the cells. This consists of two valves, 
which are usually more or less convex, and be- 
tween which the living protoplasm with its cellu- 
lar envelope is enclosed, very much as a clam or 
any other bivalve mollusk is enclosed in its shell. 
But the siliceous shell of the diatom is transpar- 
ent, and permits us to see plainly the endochrome 
within. These siliceous valves are the “ diatoms” 
with which microscopists are familiar in mounted 
preparations, and they will always be favorite ob- 
jects, both on account of the beauty and variety 
of forms which they present, and because the 
delicate markings upon some of them are used 
as tests of the resolving power of objectives. 
These markings are in the form of bands, lines, 
dots, etc. They can seldom be distinguished in 
