NEW FORMS OF MARINE DIATOMACEZ. 541 
metrical. One-half is always more or less larger than the other, and the amount 
of surface on each side of the median line is unequal. This does not occur in any 
other panduriform Navicula, except only occasionally in NV. didyma, which can- 
not be confounded with the present species. NV. splendida, N. incurvata, N. 
Musca, and N. (or P.) Pandura, are all remarkable for symmetry. In addition 
to this want of symmetry, which is invariable, it may be stated that, although 
several Naviculee, and even some of the panduriform group, vary a good deal in 
shape, there is no species which is more uniform in this respect than NV. Bombus. 
4. Navicula Smithii, var. @, fusca, fig. 15. A careful study of very numerous 
specimens, both of this form, and of that which I take to be the typical NV. 
Smithii, has now entirely satisfied me that N. fusca is truly a variety of WV. 
Smithii. But it must be added, that this, like the fresh-water JV. elliptica, is one 
of the most variable species, not only in form, but also in the striation, which 
varies from what may be called fine to exceedingly coarse; in colour, which 
varies from colourless to dark brown; and in general aspect,—/V. Smithii being 
usually destitute of the remarkable longitudinal ridge or shade on each side of 
the median line, so conspicuous in JV. fusca. In all these points, a perfect grada- 
tion may be traced without difficulty. 
5. Navicula Smithit, var. y, nitescens, fig. 16. Having found this form abun- 
dantly in one density, I have now come to the conclusion that it is no variety, 
but a distinct species. I find it perfectly uniform in all its characters, and the 
remarkable peculiarity of the median line, which is invariably a broadish white 
line with perfectly parallel sides ; while that of NV. Smithii, including NV. fusca, 
is always doubly conical, being much broader in the middle, and forming a very 
acute point at each apex, seems effectually to separate it from that species. The 
shining aspect of the strize is also peculiar. 
6. Navicula Smithii, var. 6, suborbicularis, fig. 17. This form has also occur- 
red abundantly, and I am now able to state that it is so uniform in its characters, 
and so peculiar in its aspect, that it must be admitted as a distinct and well- 
marked species. The only variation, except one to be presently mentioned, is in 
size, as it now and then occurs of twice the length of the figure, or even more, in 
which case it is more oval in shape, though always very broad. But the peculiar 
structure about the median line, giving the appearance of two white, elliptical 
bands meeting in the nodule, or of one long elliptical band, suddenly constricted 
in the middle, seems to be quite invariable, and sufficient to distinguish it. The 
fact, also, that the striz are hardly visible, except on a broad marginal band, 
where they are very conspicuous, having the shining aspect of those in J. nites- 
cens, though coarser than in that species, as well as the permanence of its very 
peculiar form, seem to indicate that it ought to be separated. Neither in this 
VOL. XXI. PART Iv. 7F 
