BRITISH DIATOMACEjE. 41 



The bizarre forms, produced by the capricious mode in which the partial 

 separation of the frustules in the chains is effected, have also occasioned con- 

 fusion ; but a little study of specimens from various localities, and collected 

 at different seasons, will satisfy the student of the impropriety of relying upon 

 these accidental peculiarities as the bases of specific distinction. 



The present may be distinguished from the last species, not only by its 

 smaller size, but by the inflation of the extremities of the valves. In 

 D. grande the apparent enlargement of the extremities is produced by a con- 

 striction ; in D. elongatum the extremities in the mature frustules are abso- 

 lutely wider than any other portion of the valve. 



Plate XL. 311. and PI. XLI. 311. Var. /3. PI. XLI. 311/3. Var. y. 

 PI. XLI. 311 y. Var. a. Supp. Plate LX. 311 3. 



4. Diatoma hyalinum, Kiitz. Filaments of numerous frustules ; 

 valve linear- elliptical, extremities somewhat acute ; striae obscure. 

 Length of frustule -0008" to -0052". Breadth of V. about -0003". 

 v.v. 



Kiitz. Bacill. xvii. 20, et Diatoma vitremn, Kiitz. Bacill. v. 7, xvii. 19, xxiv. 1 . 

 ad specim. authen. quse dedit amic. De Brebisson. Striatella t&nice- 

 formis, var. a, Ralfs, Trans. Bot. Society of Edinburgh, vol. ii. p. 22 ? 



Marine. Jersey, Aug. 1852 ; Larne, July 1853 ; Newhaven, Sussex, Aug. 

 1853, W. Sm. Saltcoats, Oct. 1853, Br.Arnott. 



Plate XLI. 312. 



5. Diatoma minimum, Ralfs. Filaments of two or three frustules ; 

 valve elliptical ; extremities rounded ; striae obscure. Length of 

 frustule -0002" to -0004". Breadth of V. about -00015". v.v. 



Ralfs in Trans. Bot. Society of Edinburgh, vol. ii. p. 20 ? 



Marine : parasitic upon frustules of Surirella Gemma. Hove, Sussex, Sept. 

 1853, W.Sm. 



I cannot refer this form with certainty to Mr. Ralfs' species, not having 

 seen specimens of the latter ; the present, however, agrees with Mr. Ralfs' 

 description, " Frustules very minute, about twice as long as broad." 



Plate XLI. 313. 



