Mr. J. Ralfs on the Dlatomaceje. 455 



and can find in it notliins: different from the common foiin. I 

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

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



Analysis. 



fFrustules with two or more striae on their front surfaces, 



interrupted in the centre by a canal 4 



Frustules without striae on their front surfaces 2 



2 f Frustules convex, thickest in the centre vidgare. 



* \ Frustules plane, the ends the thickest 3 



{Frustules linear tcnue. 

 Frustules before separation in contact only at the angles 



of the dilated ends elomjcdum. 



. TMarine; canal not inflated marinum. 



' \ Aquatic ; canal inflated 5 



r Frustules not more than twice as long as broad ; stria 



. J several on each side Jlocculosum, 



■ I Frustules four times (or more) longer than broad ; strife 



L two on each side fenestralum, 



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

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

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

 7. -D. mininmm. 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 colom* 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 ha\"ing longitudinal striae. Its character ap- 

 proaches closely to that of TesseUa, 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 striae. 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 striutuhnn, Grev. in Br. Fl. ii. p. 40.5. Achnanthes arcuata, 

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

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

 Ag. Syst. p. 7. 

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



