Sr DESMIDS AND DIATOMS. 
sions, or elevations, and frequent areolations, circular or 
hexagonal. This is a very general character of all the 
Diatomace, and is due to the deposition of layers of 
silex variously disposed within or between the different 
membranes which enclose the cell, and which, remaining 
persistent and retaining all their delicate sculpturing after 
the removal of the organic contents, are the portions 
which become fossil and which, even in recent gatherings, 
are the chief features used for the discrimination of the 
species. Frequently several different modes of ornamen- 
tation are visible upon the same shell, and are possibly 
situated in its different layers. The exact position, and 
the number of such membranes, and the nature of the 
markings, are still subjects of discussion among the stu- 
dents of these forms. The presence of this silex, how- 
ever, is the fact of most interest in their structure, 
whether they be regarded in a strictly scientific, or in an 
economical point of view. ; 
Among the discoid forms may be enumerated those 
with a circular outline, and circular or hexagonal areo- 
lations (such as Coscinodiscus, Pl. 13, fig. 34; Craspedo- 
discus, etc.), those with a circular valve divided into 
partitions by radiating lines (as Axachnodiscus, fig. 29), 
_ those with a simple disk, but united into continuous fila- 
_ ments (as Podosira, fig. 25; Melosira, etc.), those with a 
more or less circular outline, but with the surface pro- 
jecting into spines or processes which seem to connect 
adherent frustules (as in Biddulphia, fig. 31; Eupodis- 
cus, etc.) ; or, lastly, of forms not truly circular in out- 
line, but really modifications of the circle, and approach- 
ing that shape by insensible gradations (as Triceratium, 
fig. 27; Amphitetras, fig. 36; Campylodiscus, Surirella, 
ete.). Asa rule, the circular forms may be compared to 
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