DESMIDS AND DIATOMS. 513 
tolerably flat thin pill boxes, or, still better, to a common 
watch. One genus, indeed, the LTyalodiscus, when mag- 
nified over 2,500 diameters, bears a striking resemblance 
to the covers of an engine-turned watch, two sets of deli- 
cate dots radiating in eccentric curved lines from the cen- 
tre towards the periphery, and giving to the shell, under 
bright illumination, a truly exquisite appearance. Some 
have the disks so convex as to make a nearly spherical 
figure, others have the surface depressed at the centre, 
while others again are beset around the margin with a 
glittering row of spear-like points. The imagination can 
scarcely picture a form of beauty which does not find a 
counterpart among these most wonderful of Nature’s me- 
dallions. 
The multiplication of the Diatoms, like that of the 
Desmids and other unicellular Alge, takes place accord- 
ing to one or the other of two modes, either by simple 
cell-multiplication, the original frustule dividing into two 
which again subdivide, or else by a true generative pro- 
cess, and the formation of Sporangia. The first method 
is exceedingly common, so much so, indeed, that we can 
Scarcely find a specimen in which the process is not just 
ended, or in some stage of advancement. This multi- 
Plication takes place by the gradual enlargement or 
Widening of the “connecting membrane” before alluded 
to. (See Pl. 13, figs. 31 a, and 23 a.) Nearly at the 
beginning of this process the contents of the cell are di- 
vided into two portions, while the lining or inner mem- 
brane of the parent cell becomes doubled inwards in an 
annular ring about the whole circumference along the line 
of division. This infolding membrane continues to ad- 
vance, until a nearly complete division has taken place 
of the old cell into two new ones, the two new contiguous 
AMERICAN NAT., VOL. I. 65 
