142 
Wallich, on the Diatom-valve. 
structure consists in a portion of the connecting zone being 
sometimes projected under tlie surface of the valve, and faintly- 
impressed with its markings, as we find to be the case in 
Epithemia gibba, Stauroneis pulchella, and Nitzschia specta- 
bilis. In Stauroneis pulchella and Cocconeis placentula, the 
peculiar wavy appearance which is superadded to the valvular 
structure would appear to be due to this cause. 
I cannot better describe the markings on the valves of 
Pleurosigma formosum and P. Balticum, than by comparison of 
what is seen in the first to a wafer-stamp, or neck of a gun- 
stock; whereas, in the last, we have the form of marking 
that would result, were an impression taken of a wafer-stamp, 
in which the rhomboidal figures were replaced by flattened 
four-sided pyramids. In both cases there are four facets, 
inclined at a moderate angle to the plane of the surface ; and 
two of the four sets of lines they exhibit can be brought into 
focus more readily than the other two. The reason of this is 
obvious. The diagonal series in P. formosum, and the rect- 
angular in P. Balticum, being arranged strictly on the same 
plane, are capable of being brought into accurate focus simul- 
taneously. They constitute the thinnest portions of the valves. 
Whereas the longitudinal and transverse series in the first 
species, and the diagonal in the last, being constructed of a 
series of short zigzags following the rise and fall of the 
facetted portions, cannot be brought into focus at all points 
with equal exactness ; and form the thickest portions of the 
valve. The distance, moreover, between the several sets of 
lines being difi*erent, the closer series are more difficult of 
resolution. The first-named cause is, however, by far the 
most powerful. 
Without taking upon myself the unnecessary task of prov- 
ing a negative, I would briefly state my reasons seriatim for 
rejecting, as wholly inconclusive, the arguments cited in 
proof of the hexagonal ^tvuGiMveoi Pleurosigma angulatum^and 
also the evidence based upon the photographic representation. 
The analogy derived from what is seen in Triceratium and 
Isthmia has been shown to be fallacious. 
It is admitted by the advocates of the hexagonal structure 
that, under imperfect adjustments of the microscope, " hexa- 
gonal dots ^' may be made to appear quadrangular or trian- 
gular ; and that those dots which cannot be conceived to be 
really hexagonal may be made to appear so. 
In Pleurosigma Balticum or P. hippocampus, by imperfect 
adjustments, the appearance of hexagonal structure may be 
produced quite as vividly as it can be made to appear in P. 
angulatum. 
