RECENT DIATOMACES, 91 
organic substances for their support ; vegetables derive their 
sustenance from inorganic matter. 
Recent Diatomacem®. Plate IV.—To familiarize the 
reader with the nature of these vegetable organisms, a few 
recent species are represented in Plate JV., coloured as they 
appear when alive, under the microscope ; the figures are 
magnified as expressed by the fractions. 
Xanthidium. Plate IV. figs. 1, 2, 3, 4, 5.—The case or 
frustule of this genus consists of a hollow, siliceous globe, 
beset with spines. The increase of the Xanthidia by self- 
division, produces the double appearance in the figures, all 
of which are in the progress of separation.* 
Pyzxidiculum. Plate IV. fig. 2.—The case is a little 
saucer-shaped box, and is invested by a membrane. 
Bacillaria. Plate IV. fig. 6.—A simple siliceous frustule, 
of a prismatic shape, forming a brilliant chain, which often 
appears in zigzag, in consequence of incomplete self-divi- 
sion. An immense number and variety of forms are placed 
in this family by Ehrenberg, with a multitude of generic 
and specific names. The fresh-water species inhabit every 
pond and lake, and the marine every sea. Fossil species 
are equally abundant. 
Cocconeis. Plate IV. fig. 7.—This is a very elegant type; 
the frustule consists of a simple siliceous case, with a central 
opening ; it never occurs in chains like the former. It has 
been found fossil near Cassel. 
Navicula. Plate IV. figs. 8, 9, 14, 15.—The plants of 
this genus are free, and float in the water apparently by the 
agency of cilia. Their case is a boat-like envelope with six 
* The organisms so abundant in the flint and chalk, and which 
were referred by M. Ehrenberg to this genus, and consequently 
described under the name of Xanthidia by myself and others, are 
certainly in nowise related to the recent forms: they are flexible 
envelopes, and probably belong to zoophytes; as will be shown in the 
sequel. 
