NOTES ON THE STRUCTURE OF THE DIATOMACE®., 73 
ground (if the valve has its exterior towards the observer), the 
areolz catch the light, and would convince, I think, even the most 
sceptical observer, that they are not openings, but either pits or 
spherules, such observations depending upon the side of the valve 
towards the observer. 
Figs. 27 and 28, drawings of Pixnularia, by Dr. Walker Arnott, 
evidently from some portions of broken valves, giving an idea 
that the ‘“‘coste” of the Prznularia are cavities in the interior of 
the valve. Since I found the drawings, I have hunted over many 
slides in search of broken valves to bear out the point, and have 
found but few instances of that conformation, on account of the 
difficulty of finding portions of the valve in the same position. An 
appearance of depth of the coste may be got by oblique light, as 
in Fig. 29, on almost any slide. 
coos 
Fig. 29. Beam of light from mirror. 
M. J. H. Fl6zel* has made sections of Pleurosigma angulatum, 
Balticum, and scalprum, and in his illustrations has given us 
sections of the Pleurosigmas, with cavities in the interior of the 
valve, similar to those of the Pinnularia, by W. Arnott, with mag- 
nifications from 1,920 to 5,000 diameters (Figs. 30, 31, 32, 33): 
Prof. W. Smith, (Syn. Brit. Diat., Vol. 1, p. 61) has foreshadowed 
the idea (1856) :—‘“Some observers having considered these 
appearances of strize to arise from series of perforations, and others 
from rows of beads, or minute elevations. With the latter, I have 
been disposed to coincide, until, aided by the careful manipulation 
and excellent object glasses of Mr. Richard Beck, who has shown 
me the hexagonal outline of these supposed beads, I have been 
* Arch. Miter. Anat. vi., 1870, p. 480, etc. 
