MICROSCOPY. 315 
pletely illustrated as the stage is resolved, in whatever direction 
the beam of light may fall. 
Stauroneis.— Some of the larger varieties of S. pheenicenteron 
are covered with hexagonal areole easily exhibited with central 
daylight. The projecting points of the fractured partitions be- 
tween the hexagons may be observed. 
Pleurosigma angulatum.—Hexagons. The line of fracture gen- 
erally running around them, but quite often through them. 
Pleurosigma Balticum, —A drop of water slowly advancing by 
capillary attraction shows this shell to be covered with squares, 
and proves that both sets of lines forming the boundaries of the 
squares are on the same surface of the valve; and the appearance 
presented by an air bubble on the other side proves that surface 
to be smooth. i 
Pleurosigma formosum.—Near the ends of the frustule it is 
easy under certain adjustments of the light to make it appear like 
a checker-board with alternate bright red and green squares. 
Double rows of green and red beads alternating may be seen on 
this as well as on other species of the same genus. (Dr. Pigott 
in M. M. Journal.) When we resort to central light from a white 
cloud, and thus lessen the liabilities to-err caused by refraction, 
diffraction, decomposition of light, and oblique projection of shad- 
ows, the conclusion is arrived at that these various appearances 
are caused by two sets of intersecting diagonal ridges, the finer 
ridges running up and down over and between the coarser, and 
subject to considerable variation even on the same frustule. This 
theory would also account for the “ beads ”(?) being of different 
Colors, and the same “ beads” changing color when the focus is 
changed. We see in many of the mollusks shell-markings of a 
similar character. 
Concluding remarks.—It would seem that the perfect box-like 
form of the shells of the Diatomaces and their elaborate orna- 
Mentation would exclude the idea of a blind process of chemical 
crystallization. Analogy should teach that they are secreted for a 
protective covering for the tender animal-like plant, as among 
higher forms. If this is true the surface markings ought to be so 
distributed as to give additional strength to the shell without 
greatly adding to its weight. It would also be expected that 
some of the larger shells would be perforated with holes. This 
idea, of course, would have to admit into the discussion consid- 
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