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of Silex, of which their external structure is in great measure 
composed. The occurrence of this mineral in plants is not confined 
to the Diatomaceze, but is found in greater or less quantity in many 
other plants, especially in the Equisetaceze and the grasses. But its 
presence in great quantities in this group constitutes one of its 
most distinctive peculiarities. 
The cell, or frustule, as the individual diatom is called, is 
composed of two symmetrical, more or less convex, siliceous 
valves, or plates, connected with each other at the edges. They 
have not inaptly been compared to bivalve shells—the mussel, for 
instance. Interposed between these valves, and connecting their 
edges, there generally exists a third segment, forming a band or 
zone, and called the cingulum, or raphé. A common pill-box 
affords an excellent illustration of the general arrangement of these 
parts in the diatom, which is really a microscopic box of surpassing 
beauty. The lid and the bottom of the box correspond to the 
valves in the diatom; and the cylinder that intervenes between 
the upper and the lower discs of the apothecary’s casket is the 
exact counterpart of the connecting membrane of the diatom. 
The hoop, or raphé, is of the same material as the shells it unites 
together, and it plays an important part while the diatom is under- 
going subdivision. Attached to, and lining the interior surface of 
the siliceous shell, is a membrane of great delicacy, composed of a 
substance called cellulose, and known as the primodial utrical. 
Within the cavity formed by this membrane and the siliceous 
exterior, is contained a soft mucilaginous substance. 
At certain seasons, or under special conditions of the diatom, 
a number of granules or globules make their appearance in this 
contained material. These, from their refractive nature, and 
behaviour under re-agents, appear to be of an oily character. The 
endochrome, as the cell-contents are called, is sometimes of a 
bright green ; in a few species, more generally it is of a chocolate- 
brown colour. This colour is due to the iron present in its 
substance. When the diatoms occur in considerable quantities 
—which they frequently do in slowly-running streams—the colour 
they impart to the surface they cover, enables the unaided vision 
