452 Mr. J. Ralfs on the Diatomaceae. 



a kind of mucous hinge which is easily seen in this state of the 

 plant. 



Having submitted my observations to Mr. Berkeley, I had the 

 pleasure of finding that he concurred in my views, and that he 

 has long considered the longitudinally connected species as very 

 doubtful. 



The frustules are sometimes, on the contrary, broader and not 

 thrown back. This state I believe is the Diatoma sulphurascens 

 of Agardh's ' Conspectus/ It differs from Diatoma elongatum in 

 having the sides straight, and thus before the frustules separate 

 they are in contact along their entire length. Different however 

 as D. elongatum appears to be, I am doubtful whether it may not 

 be a form of this species. 



3. D. elongatum, Ag. Frustules plane, several times longer than 



broad ; front surface with the ends dilated. D. elongatum, Ag. 



Syst. p. 4 ; Kutz. in Linnsea 1833, p. 582. t. 17. f. 65. Bacillaria 



elongata, Eh. Infusoria, p. 198. pi. 15. f. 5 ; Pritchard's Infusoria, 



p. 218. f. 169. 



I notice this very doubtful British species chiefly in order to 

 call attention to it, and because it has been confounded with the 

 last, of which I am by no means certain that it is not a variety. 



I have seen no authentic specimens, and am only acquainted 

 with it from the descriptions and figures in Kutzing and Pritchard. 



Mr. Borrer informs me that he believes he saw a few scattered 

 frustules of it among other Algse gathered at Henfield, Sussex ; 

 and in a mass of Diatomaceous plants sent me from Shropshire 

 by Mr. Leighton, I observed what I supposed might be fragments 

 of this species. 



As the frustules are narrowest in the centre and dilated at the 

 ends, and before separation are in contact only at the angles, this 

 would seem to be a sufficient distinction, but I have received some 

 specimens of D. tenue which lead me to doubt whether the frus- 

 tules do not become dilated at the ends in the same way as often 

 occurs in Frustulia Ulna. In a specimen of D. tenue gathered near 

 Tunbridge Wells by Mr. Jenner, I found mixed up with the com- 

 mon state a few threads with somewhat longer frustules, some of 

 which had the sides parallel, and others in the same chain exactly 

 resembled the figures of D. elongatum, PI. IX. fig. 2. 



I observed the same appearances in a less degree in a plant found 

 by myself near Bangor, N. Wales, and considered by Mr. Shuttle- 

 worth to be the D. tenue of Agardh. 



A specimen sent me by Mr. Hassall, gathered in Scotland by 

 the Rev. D. Landsborough, presented still more curious forms. 

 Although most of it seemed to be the true D. tenue, a few frus- 

 tules not only presented the form described above, but some of 

 them were dilated at one end only ; and the next frustule was either 



