MAX SCHULTZE, ON THE DIATOM-VALVE. 127 
though some of these solid spherules may also arise from 
deposition on the outer surface of originally very minute 
spherules. 
Passing over the extremely various and often very deli- 
cate structures detected by microscopic examination among 
these siliceous bodies—all of which, moreover, are referable, 
in one way or another, to the fundamental type above de- 
scribed—the author proceeds to discuss the question of the 
structural relation between these artificial pellicles and the 
valves of diatoms, many of which exhibit in their markings 
so close a resemblance. 
The similarity between the finely marked diatom-valves 
which exhibit three sets of lines—as, for instance, in Pleuro- 
signa angulatum, &c.—with some of the similarly finely 
marked artificial pellicles (as shown in figs. 7 and 8) is so 
great, that at first sight the sculpturing would seem to be 
identically the same. In the one case, as in the other, a 
central or direct illumination brings into view minute points, 
disposed in regular rows, whilst oblique illumination pro- 
duces three sets of lines, cutting each other at an angle of 60° or 
120°. In the one case, also, as in the other, with central illu- 
mination the appearance alters according to the focal distance at 
which the object is viewed ; sometimes regular hexagons being 
brought into view, whilst at another merely minute serial 
dots are visible. But the marking, he states, in Pleuro- 
sigma angulatum and allied forms comes so close to the 
boundary of the recognisable, that a clear perception of the 
systems of lines or elevations by means of central illumination, 
and without the aid of artificial illumination—as by a con- 
denser, &c.—is possible but with few microscopes. So far 
as he is aware, this object can be effected only with the most 
powerful combinations constructed by Amici, Nachet, and 
Hartnack, Nos. 9 and 10, & immersion, or more recently by 
the latter’s No. 9 without immersion. In fact, under such 
circumstances, an investigation with respect to the true fun- 
damental cause of so obscure a marking may be termed bold. 
Is it due to pyramidal elevations on the surface, as on the 
thicker pellicles (fig. 5), or to depressions or conical hollows ; 
or is the appearance due to some totally different structure 
in the substance itself of the valve, and having nothing to 
do with differences of relief on the surface ? 
The answer to these questions has been often sought, but 
it has by no means been unanimous. The only thing that is 
quite positive is that the marking in question is due to actual 
differences of relief on the outer surface of the valve. 
