Mr, J. Ralfs on the Diatomaceae. 455 



and can find in it nothing different from the common form. I 

 have seen no specimen of D. brachygonum, Carin., but there is 

 nothing in the description to distinguish it from D. marinum. 



Analysis. 

 r Frustules with two or more striae on their front surfaces, 



l.i interrupted in the centre by a canal 4 



[Frustules without striae on their front surfaces 2 



2 ( Frustules convex, thickest in the centre vulgar e. 



* \ Frustules plane, the ends the thickest 3 



["Frustules linear tenue. 



3. "S Frustules before separation in contact only at the angles 



L of the dilated ends elongatum. 



. ( Marine; canal not inflated marinum. 



' \ Aquatic; canal inflated 5 



[* Frustules not more than twice as long as broad ; striae 



t, J several on each side , flocculosum. 



'J Frustules four times (or more) longer than broad ; striae 



I two on each side fenestratum. 



I shall place for the present in an appendix a recently disco- 

 vered plant, the characters of which, on account of its minuteness, 

 I have not yet been able to determine with any certainty. 



7. D. minimum. Frustules very minute, about twice as long as broad, 

 nearly colourless. Tetraspila minima, Shuttle worth in lit. 

 On Conferva uncialis, Penzance. 



This species is certainly distinct from any of those described 

 above. I can find no strise on either surface ; but, if these exist, 

 its pale colour and small size well distinguish it from Diatoma ma- 

 rinum, the only other marine species I am acquainted with. 



Striatella, Ag. (Ehrenb.) 

 Filaments attached by a stipes ; frustules cohering by the angles, 



longitudinally striated. 



This genus differs from Achnanthes by its frustules cohering 

 at their angles and having longitudinal strise. Its character ap- 

 proaches closely to that of Tessella, Eh., from which it differs only 

 in the stipitate filaments. 



Filaments attached by a stipes at one of the lower angles of the 

 basal frustule. 



1 . S. arcuata, Ag. Stipes shorter than the frustules ; filaments elon- 

 gated ; frustules with seven or more longitudinal series of short 

 transverse strise. Ag. Consp. Diatom, p. 61 ; Harv. Br. Alg. p. 199; 

 Eh. Die Infus. p. 229. t. 20. f. 6 ; Pritch. Infus. p. 223. f. 203. 

 Diatoma striatulum, Grev. in Br. Fl. ii. p. 405. Achnanthes arcuata, 

 Kutz. Syn. Diatom, p. 574. Ach. unipunctata, Berk. Br. Alg. p. 39. 

 t. 15. f. ii Conf. striatula, Eng. Bot. 1. 1928. Fragilaria striatula, 

 Ag. Syst. p. 7; 



Plate IX. fig. 6. a, front view ; b and c, lateral views. 



