126 MAX SCHULTZE, ON THE DIATOM-VALVE. 
all are reconcilable with the existence of conical, acuminate 
elevations, such as are represented in the side view in fig. 9. 
Another set of appearances may be illustrated by figs. 6 and 
6 a, the latter being the profile view. These are found when 
the elevations, instead of being pointed or acuminate, are 
rounded and obtuse at the summit. The elevations in this 
case, when viewed at the proper focal distance, are distin- 
guished by their presenting one or more concentric rings. 
The appearances presented in both cases—that is to say, in 
the pointed or acuminate elevations, and in the rounded or 
obtuse ones—are all explicable upon the assumption of their 
being composed of superimposed lamine, gradually diminish- 
ing in size. 
It is to be observed that, however constant is the occurrence 
of these elevations on the outer surface of the vesicles, the 
inner surface, more especially in the thicker-walled ones, is 
always smooth and even. 
What has been said above with respect to the structure of 
the siliceous vesicles suffices to afford a notion as to the 
mode in which their gradual formation occurs ; and repeated 
observations leave no doubt that this takes place in the fol- 
lowing way :—The first deposition of silex is in the form of 
minute spherules or lenticular particles, which usually aggre- 
gate themselves into pellicles, constituting, for the most part, 
spherical-or cylindrical vesicles, which, however, never, or at 
any rate very rarely, appear to be entirely closed. 'The size of 
these siliceous particles varies extremely, for reasons which 
are not, in all cases, easily determined, but which have mani- 
festly some relation to the rapidity with which the evolution 
of the fluoride of silicium takes place. Fig. 4 represents a 
transverse section of a portion of a vesicle composed of the 
larger kind of lenticular particles. ‘These particles at first 
project equally on both surfaces; but this condition is soon 
altered, owing to the circumstance that the continued depo- 
sition of silex does not go on uninterruptedly over the whole 
outer surface, but only or chiefly on the elevations, which, 
consequently, assume the form of convex lenses, which, as 
they increase in size and interfere with each other, gradually 
acquire hexagonal outlines. The pellicle, however, increases 
in thickness internally as well, but on this aspect the deposi- 
tion goes on uniformly over the surface, the consequence of 
which is that the hollows are gradually filled up and the 
elevations obliterated (fig. 3). It seems probable that this 
internal thickening may go so far as wholly to obliterate the 
cavity; and thus a solid spherule of silex is formed, pre- 
senting, as above stated, the optical properties of hyalite ; 
