128 MAX SCHULTZE, ON THE DIATOM-VALVE. 
Wenham* was struck with the happy thought of preparing 
galvano-plastic impressions of diatoms, which were in his 
hands perfectly successful, and represented inpressions 
of the systems of lines or dots. For this purpose he 
selected only two species, Pleurosigma balticum and P. hippo- 
campus, both of which are ‘among the very finely marked 
diatoms, though far more easy resolvable than Pleurosigma 
angulatum. They differ from the latter also in the circum- 
stance that, when we view the systems of lines more particu- 
larly as brought out by oblique light, in the former two 
species only two sets will be perceived, crossing each other at 
a right angle, whilst in P. angulatum three will be seen inter- 
secting each other at an angle of 60°. An explanation, 
therefore, of what is seen in the former need not be equally 
true of the latter. But the general aspect of both species, 
except in the above respect, is so much alike that no one 
could object to the application of Wenham’s explanation of 
what is seen in P. balticum to the appearance presented in 
P. angulatum. It is true that Schachtt+ has lately given a 
description of the marking which might raise a doubt whether 
the three sets of lines in question are really situated on the 
surface of the object. He says—‘“In order to display each 
set of lines, it is in many cases necessary, besides the turning of 
the stage, to alter the focus somewhat, from the circumstance 
that each of the three sets of lines belongs to a distinct layer 
in the valve, and consequently is placed a little above or 
beneath the other.”? And this he says notwithstanding that, 
as he himself allows, all the three sets may be brought into 
view at once. Further on it is said— The horizontal lines 
appear to be the deepest seated, and are, perhaps, on that 
account the most faintly marked.” The apparently moniliform 
structure of these diatom-valves, consequently, according 
to Schacht, is deceptive, and is only perceived in case the suc- 
cessive sets of lines, when, as under certain circumstances, 
they come into view simultaneously, are viewed altogether. 
Upon what conditions, therefore, he considers the striation 
really to depend remains obscure, although in a subsequent 
passage Schacht, in explaining the matter, employs the ex- 
* ©Quart. Journ. Mic. Sc.,’ IIT, 1855, p. 244. It does not appear that 
Mr. Wenham’s experiments were limited to the two species named, as he 
says that he had “obtained distinct impressions of the markings of some 
of the more difficult Diatomacer, such as V, balticum, P. hippocampus, &e., 
leaving,” he goes on to say, “no doubt of their prominent nature.” But 
whether this prominence belongs to the areole or intermediate lines does 
not appear.—Lps. 
+ ‘Der Mikroskop und seine Anwendung,’ 1862, p. 29. 
