16 
Greville^ on New Diatoms. 
median line^ as to render an observation on that organ expe- 
dient before proceeding with my description. The term 
central or median line is^ at present, used with great latitude, 
and seems to be held to include, not only the truly simple 
central line, which extends longitudinally throughout the 
valve, but also, in various instances, two additional lines 
which run close to and parallel with it. In the present case 
it becomes necessary to distinguish between them, and, until 
a better name be suggested, I shall call these additional lines 
the extra-median lines. In N. Lewisiana the true median 
line, which is very slender, passes to the base, and, as it were, 
supports the terminal nodule. The extra-median lines are 
very much stronger, incrassated, and somewhat convex oppo- 
site the central nodule, and on reaching the terminal nodules 
suddenly expand, increase in bulk, and embrace the lower 
part of the nodule exactly as a porte-crayon holds a pencil — 
a comparison which I perceive, from Dr. Wallich^s notes, we 
have both made independently of each other. The frustule 
is diaphanous, even under considerable magnifying power, 
and the striae strictly transverse and parallel, and so fine that 
they cannot be exhibited by the engraver. According to Dr. 
Lewis, they are 50 to 60 in '001", while Dr. Wallich makes 
them 85 in '001". I confess that I have been unable to 
estimate them satisfactorily. In the ^ Microscopical Journal'' 
(vol. ii, p. 155, n. ser.) is a partial reprint of Dr. Lewises 
original pamphlet, in which this species is referred, with a 
question, to N. fossilis, Ehr., as well as to N. rhomboides, a 
suggestion which does not occur in the original pamphlet 
itself. That diatom, however, has, I believe, never been 
described, and we only know it by the figures in ^ Mikro- 
geologie^ (pi. 10 I, fig. 6). Judging from these figures, it 
is a minute species, with the strise visible and highly oblique, 
II few radiating ones being very conspicuous opposite the 
central nodule. It seems quite clear, therefore, that it has 
no afiinity with the very fine and curious diatom under con- 
sideration. 
I have attempted in vain to render the frustule of N. 
Lewisiana stationary under examination, in order to obtain a 
drawing of the front view. That, however, given by Dr. 
Lewis, " linear and slightly inflated,^^ appears to be correct. 
In one or two immaterial points this species is subject to 
irregularity. The terminal nodules vary in length, and the 
forceps-like receptacle is sometimes closed upon the nodule, 
while at others it slightly expands. The size of the frustule 
is also uncertain. The specimens from Sierra Leone are the 
largest I have seen, one in my possession being nearly a 
