MANN] DIATOMS, THE JEWELS OF THE PLANT-WORLD 51 
the most amazing numbers of individuals are found in the arctic 
regions. Dr. Nansen found them in undiminished abundance at the 
northern limits of his polar journey. 
Geologically the diatoms seem to have first appeared in the closing 
measures of the Middle Cretaceous ; in other words, they are, though 
low in organic complexity, comparatively recent in the scale of suc- 
cessive life-forms. The statement of Castracane, that they have been 
found in the Carboniferous Era, and the still more amazing claims 
that have been made of finding them in Devonian and even Silurian 
deposits are generally discredited. It should be remarked here that 
the precise period of entrance of these organisms should be quite 
clear, because of the prolific multiplication of individuals character- 
istic of these forms on the one hand and of the indestructible nature 
of their remains on the other. 
Diatom structure can be best understood by looking first at the 
external sxeleton or casing, and then at the living substance within 
it. Each plant, a unicellular individual, secretes for itself an external 
case or box of clear and very dense silica, consisting of two valves, an 
upper and a lower, the one slipping over the other like the lid and 
bottom of an ordinary pasteboard box, 
Figs. 8 and 9, A, B. This case is not 
of uniform shape; but among the 4,000 
or more species there can be found al- 
most every conceivable form, so long 
as the form displays symmetry on one 
or both of its axes. Thus there are 
round, square, triangular, stellate, oval, 
ovoid, crescent, sigmoid, cuneate, ba- 
cillar, etc., forms. It is evident that 
this great variety in the symmetrical con- 
tour of these structures adds considerably 
to their beauty and attractiveness. The 
variety is further enhanced by numerous 
outgrowths in the form of spines, horns 
or domes, so arranged as to preserve the symmetry of the valves 
from which they spring. The two valves themselves, which are 
with few exceptions identical in shape and markings, are carved and 
ornamented with an elegance and variety that is well-nigh incon- 
ceivable. Indeed it may safely be stated that there is hardly a kind 
of surface ornamentation known that has not been utilized in beauti- 
fying these structures. Polished beads of varying size arranged 
in radiating or concentric rows, shining bars, wavy ridges, delicate 
[Eee ae}. 
