138 
Wallich, on the Diatom-valve. 
to be viewed in one or other of its aspects, so must the 
appearance of elevations or depressions vary. At present, it 
is only in some of the more boldly- marked species that we 
can decidedly pronounce which surface of a valve is directed 
towards the observer. On this account I have adopted the 
use of the somewhat vague term, " intra-linear spaces, to 
designate those portions in which the appearances of eleva- 
tions or depressions occur, leaving the peculiar nature of 
such spaces to be dealt with in each separate case. 
If we examine the valve of any of the most boldly- 
marked species belonging to the Naviculoid group of dia- 
toms, as, for example, Pinnularia distans, alpina, or lata, we 
meet with what I conceive to be the simplest form of linea- 
tion, namely, a series of narrow, elongated, depressed 
"costse,^^ — as they are very inappropriately termed, — ar- 
ranged in transverse order on the surface of the valve, and 
rendered remarkably distinct by their superior degree of 
translucence, and the contrast they present in refractive 
power with the adjacent parts. Neither the depressed 
"costae^^ nor the intra-costal spaces exhibit any trace of 
secondary markings. In proof of the '^^costse'' being 
depressions, it may be mentioned that, whilst the median 
line and nodule and the entire margin of the valves exhibit 
one uniform colour, usually a pale rose pink, the " costse 
partake of the faint-blue tint observable in the surrounding 
field of vision ; and lastly, that, in accidentally fractured 
valves, the intra-costal spaces are left more or less entire, 
like the teeth of a comb, attached to the median portion of 
the valve, which is precisely the opposite of what would 
occur did the intra-costal bars constitute the thinnest por- 
tions of the valve, whilst the costse are the thickest. 
In the genus Navicula, we find this kind of structure 
modified, but in a slight degree ; and this, it appears to me, 
has not been clearly shown heretofore, inasmuch as the very 
term " striae,^' which is specially employed to indicate the 
structure, at once suggests the idea of projecting lines, or 
bands minutely scored across, at intervals; whereas the 
diff'erence between the markings in Pinnularia and Navicula 
consists only in the depressed spaces in the latter genus 
being so minute as to admit of their arrangement, at inter- 
vals, in linear series across the valve, and thus appearing as 
unbroken lines under the lower powers of the microscope. 
In this instance, the definition, resolvable into circular 
dots,^^ is strictly accurate. 
In the monstrosity which has been dignified with the 
name of " Surirella craticula/^ we at times meet with the 
