NEW FORMS OF MARINE DIATOMACE^. 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. PI. X., figs. 40 and 40 b. 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. Striae 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 Smith, as having 

 occurred sparingly in a dredging made by Mr G. Barlee, off the coast of Skye, 

 and detected by Professor Williamson. It was referred by Professor Smith, but 

 doubtfully, from the absence of the S.V., to Himantidium. The S.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. I would indicate one curious character, that the striae, on the F.V. 

 seem to pass across the intervals between the adjacent frustules. This seems 

 also to have been observed by Professor Smith, 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 

 striae, 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 (?) ), n. sp. PI. X ., figs. 41 

 and 41 b. 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. 6 S 



