DIATOMACE.E 189 
under Chlorophycee, notably the Desmidiew, the 
Peridinice (which have also a peculiar colouring- 
matter), and, it may eventually prove, the calcareous 
Coccospheres and Rhabdospheres. The Diatoms 
have a very extensive literature of their own, from 
the fact that their great beauty and universal dis- 
tribution have made them in a singular degree the 
objects of special study. 
General Characters—The plants are very minute 
unicellular, with the walls strongly silicified, and 
composed of two overlapping halves or shells called 
valves. The valves overlap, the one above the other at 
the edge, much as the lid of a cardboard box overlaps 
the box itself, and this overlapping edge is called the 
girdle. Each individual, consisting of two valves, is 
called a frustule in the special literature of the sub- 
ject. The colouring-matter is chlorophyll masked 
by a brownish substance called diatomine, readily 
soluble in alcohol, and resembling the peculiar 
colouring-matter of the Phawophycee. Many Diatoms 
are endowed with a power of independent movement. 
The reproduction affords highly distinctive charac- 
ters. After a series of successive bipartitions, in- 
volving a gradual diminution in the size of each new 
generation, the original dimensions are regained by 
the formation of an auxospore by various methods, 
some of them involving conjugation, but not 
between motile gametes. 
The Thallus—The individual Diatoms live either 
singly or in chains, and they may be wholly free, or 
borne on gelatinous stalks, or enveloped in a 
gelatinous mass. The fresh-water forms do not 
