132 
WallicHj on the Diatom-valve. 
it surely authorises us to accept as most closely allied in 
structure those forms which^ by universal consent^ have been 
arranged side by side. We are sure to meet with stronger 
and more reliable analogies amongst the several species of 
the same genuSj than amongst those of distinct genera. The 
statement of the one fact is but a statement of the other. 
And^ for this reason^ it is obvious that, in microscopic inves- 
tigation, we are warranted in resorting to the most strongly 
marked and most readily observed species of a genus, when 
we attempt to draw general conclusions regarding its orga- 
nization. 
I allude just now more particularly to the genus Pleuro- 
sigma, of which a species, P. angulatum, has been handed 
down from writer to writer, both in Great Britain and abroad, 
as the one, par excellence, presenting us with typical struc- 
ture. That it does exhibit structure identical in every essen- 
tial particular with that observable in the rest of the species 
of that genus is undeniable. But, although by no means a 
difficult object for a quarter-inch lens of good construction, 
the markings on its valves are much more minute and 
delicate than is the case with other species ; and accordingly 
the risk of misinterpretation, when it is viewed under higher 
powers, is necessarily augmented. I shall therefore select 
two other forms as typical of the rhomboidal and quadran- 
gular structure, in which the characters can be made palpable 
both more easily and more distinctly. 
Before entering, however, on a description of the structure 
in Pleurosigma, I would briefly point out the manner in 
which it appears to me the Diatomaceous fnistule is 
developed ; and to what extent the variations of conditions, to 
which it is subject during such development, seem likely to 
affect the siliceous deposit. 
In the first place I would mention, that there is no evidence 
of growth proceeding continuously in the diatom-valve — if 
by growth be meant the increase and extension of the entire 
structure in every direction, as occurs in the higher orders of 
the vegetable kingdom. Some writers have endeavoured to 
account for the variable character of the striation in certain 
species, by asserting that, although the number of striae in a 
minute fractional part of an inch is subject to considerable 
variation, the total number of strise on the valves of all indi- 
viduals of the same species or varieties is not liable to the 
like amount of deviation. In other words, they consider 
that the siliceous valve is capable of continuous growth ; and 
that, whilst no fresh striae are added to the valve, the distance 
between the several strise may be augmented. Others again 
