OF THE CHARACTERISTICS OF THE DIATOMACEAS. 43 
are not only very beautiful objects in themselves, but also 
of great importance in the determination of species. They 
are dependent on the presence of certain ridges, called costa?, 
which are connected with an inner membrane, and also of 
numerous points, more or less fine, which dot the exterior 
membrane. These points, though sometimes scattered 
about without order, are more usually seen to be in regular 
series. Under a low power, the rows of points appear as 
lines extending across or along the surface of the valve. 
They are then called stria?,* and, in determining species, it 
is usual to speak of them under that name, although with 
a very high power most, if not all, of the stria? are resol- 
vable into dots or points. The differences arising from the 
fineness, approximation, and position of these stria? con- 
stitute the characteristics employed in fixing species. The 
costa is always visible, even when the valve is immersed in 
water ; but to bring out the striae and points clearly is a 
much more difficult matter. Before all things the contents- 
of the frustule must be entirely removed, or the latter will 
not become transparent. Next to that it must be thoroughly 
dried ; for, if the hollows on the surface of the valve con- 
tain any moisture, the passage of the light is interfered with, 
and the markings rendered indistinct, if not invisible.. 
Lastly, the resolution of the striae requires not only a 
really good microscope, but also extremely careful and de- 
licate manipulation, especially as regards the illumination 
of the object. 
In order to dry the frustules quickly and with safety, and 
to free them from endochrome and other soft substances, they 
may be exposed for a few seconds to a red heat, which will 
entirely destroy all organic matter, will dry up any re- 
maining moisture, and leave the silicious valves bright and 
transparent. The heating is accomplished by placing a 
* In fig. 48, the costa or vitta, or median line, is the central line 
marked a , d. The striae are the lines radiating from the centre to 
the hoop on each side of the costa. The central spot is named the 
nodule. 
