350 J. W. Bailey on Diatomacee. 
have been called orifices by some and depressions by others, are 
the last of all to disappear as the shell is dissolved. This mode 
of observation, beside establishing the fact that these are really 
the thickest parts of the shell, reveals many interesting particulars 
of structure in the various genera of Diatomaceee i 
large species of Pinnularia, it m e seen with even a low 
power, that the two parallel bands (separated by a canal) which 
reach from the central knob to the terminal ones, and which 
appear smooth before the application of the acid, become dis- 
tinctly striated after their surface is dissolved off, as does also the 
central spot itself, showing that striz which existed in the young 
shell are covered up and nearly obliterated by subsequent deposits. 
In Stauroneis the cross-band and the two longitudinal bands 
are the last to dissolve, and these last bands, as. in most of the 
family, appear separated by what is either a canal or a very thin 
portion of the shell. In Grammatophora the undulating lines 
are internal plates which are the last to dissolve. In Heliopelta, 
Actinoptychus, &c., the polygonal central spot is the last to dis- 
appear. In Isthmia, the spots on the surface, which at first ap- 
pear like granular projections, are in reality thin portions of the 
shell, and under the action of the acid they soon become real 
holes. The acid also proves that the larger spots at the trans" 
verse bands are really a series of large arcuate holes in the silic- 
eous:shell, and the piers of this series of arches, remain some 
time after all the rest of the shell has vanished. Many other in- 
teresting facts are revealed by the action of this acid on these 
shells, and no one can use it without learning much with regard 
to their true structure. : 
A few directions with regard to the mode of manipulation in 
these experiments will probably be nseful. As the fumes of the 
hydrofluoric acid, if they reached the lenses, would greatly in- 
jure them, I would advise experimenters (even if they have a 
micro-chemical stage) to protect the front face of their objectives 
by temporarily cementing to them a thin plate of mica by means 
of Canada balsam. ‘his can be attached or removed in a few 
moments, and completely protects the lens without materially af- 
* ‘ 2. On the cell-membrane of Diatomaceous shells.— f hydro- 
uoric acid is applied to recent Diatomacez, the silica soon dis- 
solves leaving distinct, internal, flexible cell-membranes retaining 
Ce fal 
oe 
