542 PROF. GREGORY ON NEW FORMS OF MARINE DIATOMACES. 
form nor in the preceding, have I seen the slightest trace of any tendency to pass 
into V. Smithii, or into its variety N. fusca. 
I have, indeed, lately noticed one variety of the present form, namely, a pan- 
duriform variety, agreeing with the type in size, in general aspect, and in the pe- 
culiar median line. This I shall describe and figure on a future occasion. 
I may here add, that I shall also have to describe and figure another new form 
of Navicula, occurring abundantly with the preceding ones, which at first I was 
disposed to refer, like them, to V. Smithit. But I find it so uniformly peculiar, 
that I must separate it also. 
7. Denticula (?) levis, fig. 33. I have some reason to think that I have detected 
the 8.V. of this species. The F.V. is frequent in some densities, but it would 
appear that the entire frustule is so much broader on the F.V., that it never lies 
on the S.V., and that the valves are never, or hardly ever, separated. Even when 
separate, the S.V. must be so very narrow, and perhaps so convex, that this side 
is not usuallyseen. In one case, however, where one of a group has been partly 
turned, I think I can see that the S.V. resembles in shape that of D. fulva, only 
smaller and narrower. 
I have also to add to the list of British species two forms, both remarkable, 
which occur in Lamlash Bay. 
These are,—1. Cocconeis Morrisiana, Sm., a very curious species lately found 
by Professor Situ, I believe, in a gathering from the Levant or from the Black 
Sea. 2. Pleurosigma compactum, Grev.; described and figured by Dr GREVILLE, 
from Trinidad. I propose, in a future paper, to figure these two species as British 
forms. 
I have just now been able to add to the figures, one of the very remarkable 
detached segment of Amphora spectabilis, as described in the text. It will be 
found in Pl. XIII, fig. 79 e. 
28th May 1857. 
