NEW FORMS OF MARINE DIATOMACE^. 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 N. didyma, which can- 

 not be confounded with the present species. N. 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 Navicular, 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 N. Bombus. 



4. Navicula Smithii, var. (3, fusca, fig. 15. A careful study of very numerous 

 specimens, both of this form, and of that which I take to be the typical N. 

 Smithii, has now entirely satisfied me that N. fusca is truly a variety of N. 

 Smithii. But it must be added, that this, like the fresh- water N. 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, — N. Smithii being 

 usually destitute of the remarkable longitudinal ridge or shade on each side of 

 the median line, so conspicuous in N. fusca. In all these points, a perfect grada- 

 tion may be traced without difficulty. 



5. Navicula Smithii, var. 7, 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 N. Smithii, including N. 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 striae is also peculiar. 



6. Navicula Smithii, var. 8, 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 striae are hardly visible, except on a broad marginal band, 

 where they are very conspicuous, having the shining aspect of those in N. 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. 7 F 



