NEW FORMS OF MARINE DIATOMACEA. 497 
length, the S.V. is one of the longest. The whole form has a pale, whitish aspect 
on the S.V., and the single valves are hyaline. 
This very fine and conspicuous form is frequent in Lamlash Bay, and even 
abundant in one of the Loch Fine dredgings. It occurs scattered in all the dredg- 
ings without exception. It seems to be a true Denticula. 
40. Diadesmis (?) Williamsoni,n. sp. Pl. X., figs. 40 and 406. Form of F.V. 
rectangular, narrow, with three angular expansions, one in the middle, and one at 
each end; so that there are two long, narrow elliptical spaces between the adjacent 
frustules, which occur, as in the last species, in chains of from two to twenty, and 
upwards. Margin of F.V. strongly denticulate. Length from 0-0016" to 0-008’ ; 
breadth from 0-0004" to 0:0005"._S.V. linear, narrow, straight, very slightly in- 
curved in the middle, and acuminate at the ends. Strive coarse, coarsely monili- 
form, but closely set, giving to the valve a black aspect; about 16 or 18 in 
0-001". Central and terminal nodules large, white, conspicuous. 
This remarkable and conspicuous form occurs with the last, and is even more 
abundant, especially in two of the Loch Fine dredgings. It is found in all the 
materials. The F.V. has been described and figured by Professor Smrru, as having 
occurred sparingly in a dredging made by Mr G. Barter, off the coast of Skye, 
and detected by Professor Wituiamson. It was referred by Professor Smiru, but 
doubtfully, from the absence of the S.V., to Himantidium. The 8.V. is particularly 
frequent in one of my Loch Fine dredgings, and certainly cannot belong to Himan- 
tidium. The nearest genus is Diadesmis; but I do not feel at all sure that it is 
the true one. The conspicuous nodules agree with it; but the aspect of the species 
is unlike that of any known Diadesmis. I therefore give it as such with a doubt, 
and am satisfied with having, in the meantime, ascertained that it is not a Hi- 
mantidium. 1 would indicate one curious character, that the striz, on the F.V. 
seem to pass across the intervals between the adjacent frustules. This seems 
also to have been observed by Professor Smitu, as it is represented in his figures 
of the F.V., which are accurate. I have also observed, that the shortest examples, 
which are sometimes so short as to be nearly square on the F.V. are not only 
broader on both views than the larger ones, but also devoid of the central expan- 
sion or undulation, seen on the F.V. In this state, they approach in form to the 
rectangular F'.V. of D. distans, but are at once recognised by the closeness of the 
strize, as well as by their moniliform character. In one dredging I find many long 
filaments, especially of the shorter frustules; while in another, treated precisely 
in the same way, the detached frustules, generally exhibiting the S.V., and much 
longer, are much more common than the chains. 
41. Meridion (?) marinum (or Gomphonema lineare (?)), nu. sp. PI. X., figs. 41 
and 416. Form of F.V., which occurs in chains of two, three, or four, both in 
entire apposition, or attached only by an angle at the broader end, cuneate and 
truncate at both ends, and narrow. It is marked with coarse marginal denticu- 
VOL. XXI. PART Iv. 6s 
