122 MAX SCHULTZE, ON THE DIATOM-VALVE. 
tions are sometimes rounded at the summit and circular at 
the base, might be attributed to the accidental interference 
of free hydrofluo-silicic acid, &c. But experiments to elimi- 
nate the action of this agent showed that it had nothing to do 
with the variety of appearance in the elevations. 
The subject presented no less interest as regards the ex- 
planation of the peculiar structure of the diatomaceous valve. 
Most of the manifold species of these organisms are charac- 
. terised by the presence on their outer surface of certain 
differences of relief, referable either to elevations or to de- 
pressions disposed in rows. ‘The opinions of microscopists 
with respect to the nature of this marking are divided. Whilst 
in the larger forms and those distinguished by their coarser 
dots the appearance is manifestly due to the existence of 
thinner spots in the valve, we cannot so easily explain the 
cause of the so-termed striation or punctation in Plewrosigma 
angulatum, and similar delicately marked forms. In these 
it often seems as if the appearance were due to pyramidal 
elevations, with hexagonal bases, and standing in regular rows, 
exactly like those observed in the siliceous vesicles above 
mentioned. 
In any case, it seemed likely, since the marking just 
noticed is observed to be essentially alike in many different 
species of diatoms, that its ultimate cause were to be sought, 
less, perhaps, in any organic formative process, than in the 
deposition of silex under the same laws as those by which its 
deposition in the other case is regulated: Were this ultimate 
cause shown to be crystallization, the question would be 
solved. 
But in other cases, that the secretion of amorphous silex 
also occurs in the diatomsis undeniable. The specific gravity 
of the diatoms in the so-termed infusorial earth of Liine- 
biirger-Haide has been determined by Graf. Shaafgotsch to 
be 2°2, showing clearly that in this case the silex is in the 
amorphous condition. But the diatoms in this deposit do 
not belong to those characterised by the surface marking, 
which, as has been hinted, might be referred to crystalli- 
zation. They are fresh-water forms, whilst that kind of 
marking is more especially peculiar to the marine diatoms in 
general. It may be supposed that the sea-water is favor- 
able to the deposition of silex in the crystalline condition, 
The author was induced, in consequence, to take considerable 
pains to determine the sp. gr. of the marine diatoms, col- 
lected together in considerable quantity, but in vain, owing 
to the circumstance that they are so abundantly mixed up 
