DESCRIPTION OF PLATES. 185 
cannot fail to be impressed with the immense 
period of time which must have been required 
for the formation of a deposit of this thickness. 
The plates introduced into the present volume 
will serve to illustrate the beauty and variety of 
forms presented by these interesting objects. 
In Plate XII. we have, arranged in six rows, 
ninety of these siliceous valves from various parts 
of the world, and including both recent and fos- 
sil species. The amplification is not sufficient 
(fifty diameters) to show the more delicate 
markings, and, indeed, some of the smaller spe- 
cies are only imperfectly seen (ede page 22). 
After admiring the beauty of these diatoms in 
detail, the reader cannot fail to be astonished at 
the skill of the preparator who has selected them 
from rough material collected in distant parts 
of the globe, and, has arranged them in this 
methodical manner, not only giving each its 
proper place in the Typen-Platte, but excluding 
all extraneous particles —dust— which would 
mar the beauty of the picture. 
This is only one of four squares all arranged 
in the same way and upon a single slide, where 
the space occupied is less than one-sixth of an 
inch in area, while the number of diatoms is 472. 
Some amateur microscopists in this country 
have succeeded in arranging a smaller number 
of diatoms in regular order in a very creditable 
manner ; but the patience and technical skill ex- 
hibited in the preparation of the Zypen-Platte of 
Moller are not likely to be equalled, and can 
scarcely be excelled. 
