1382 MAX SCHULTZE, ON THE DIATOM-VALVE. 
at the points of intersection, afford an appearance resembling 
that of tubercular elevations. 
But how is the circumstance to be explained that, upon 
the still further lowering of the tube, the appearance of the 
bright points corresponding to the bottoms of the depressions 
is again succeeded by that of dark ones? With respect to 
this, the author is only able to surmise that on the inner sur- 
face of the valve there is a sculpturing similar to that on the 
outer ; and, consequently, that when, by the lowering of the 
tube, the borders of the inner depressions are brought accu- 
rately into focus, the depressions themselves appear as dark 
points. And in the same way may be explained the ultimate 
indistinctness of the previously bright, punctiform markings. 
In cases where two sets of lines intersect each other at a 
right angle, as in P. balticum, hippocampus, and attenuatum, 
the disposition of the ridges at once suffices to account for 
the arrangement of the quadrangular interspaces. But it is 
not so easy to explain the disposition of the hexagons pro- 
duced by the three sets of ridges intersecting each other at 
an angle of 60° which exist in P. angulatum and its allies. 
It is most natural to suppose that they would be arranged 
like the cells in a honeycomb, that is to say, in the manner 
shown in fig. 11. An arrangement of this kind is figured 
amongst others by Carpenter and Mr. Ch. Hall.* Schacht, 
however, gives a different view of the arrangement of the 
hexagons. According to him, they are not in accurate 
mutual contact, like the cells in the honeycomb, but so dis- 
posed as to leave between them minute triangular spaces. 
These interspaces, then, might either, like the hexagons 
themselves, represent depressions, or might be elevations, and 
in this case conduce to the formation of the borders of the 
depressions. The author has never been able to see these 
triangular interspaces, and cannot coincide in Schacht’s 
opinion as to the boundaries between the hexagons being 
formed by sets of continuous elevated ridges. That the 
hexagons are arranged as figured by Mr. Hall, that is to say, 
like fig. 11, is fully established to the author’s satisfaction 
by some photographic representations procured by the aid of 
Hartnack’s combination. According to these figures,'the lines 
in each set are not continuous in a straight direction, but 
are bent at short intervals, atan angle of 120°. These bends 
are, however, so close together as to be imperceptible with the 
power usually employed in the examination of P. angulatum, 
* ‘Quart. Journ. Mic. Sc.,’ Vol. IV, 1856, Pl. XIII, fig. 2. Vide also 
* Micrographie Dictionary,’ 1856, pl. xlvii, figs. 41 and 48, the former of 
which is copied from a photograph by Mr. Wenham, 
