Mr. J. Ralfs on the Diatomacese. 109 



Plant pale brown when recent; in drying becomes grayish, 

 often with a silvery lustre. 



Filaments attenuated ; endochrome consisting of rather large 

 granules arranged in a central line running down the filament ; 

 frustules very narrow, many times longer than broad; puncta 

 indistinct or wanting. 



Ehrenberg describes the lateral surfaces as acicular with the 

 ends acute : they are so narrow that I have not succeeded in ob- 

 taining a satisfactory \dew of them ; but they appear to be narrow- 

 lanceolate, without striae, constricted near the ends, which are 

 somewhat obtuse. 



This species is probably often passed over as a state of F. pec- 

 tinalis ; but the narrow frustules, the absence of striae, and the 

 form of the lateral surfaces sufficiently distinguish it. Mr. D. 

 Moore, who obligingly favoured me with drawings and remarks 

 on many of the minute Irish Algae, has figured and described 

 this species as distinct from F. pectinalis ; he also observed that 

 it is less fragile than that plant. 



I have placed the reference to Kutzing under this species 

 rather than under F. pectinalis^ because he says, " siccitate argen- 

 tescente,^' a character far more applicable to this plant. 



Plate II. fig. 4. a, F. rhabdosoma; b, frustule, deprived of its endo- 

 chrome; c, frustule, lateral view. 



3. F. hy emails, Lyngb. Frustules broad; puncta at the ends very 

 minute ; lateral surfaces elliptic-lanceolate, with well-marked striae, 

 which terminate in distinct puncta along the margins. Lyngb. t. 

 63 ; Ag. Syst. p. 7 ; Consp. Diatom, p. 63 ; Kutz. in Linnsea 1833, 

 p. 72. F. confervoides, Grev. in Br. Fl. vol. ii. p. 403 ! ; Harv. Br. 

 Alg. p. 197." 



In freshwater pools and rivulets. Sussex, Mr. Borrer ; Llyn Pre- 

 feddyr near Barmouth, Rev. T. Salwey ; Aberdeen, Dr. Dickie; 

 Stevenston, Rev. D. Landshorough ; Ballantrae, Ayrshire, Mr. W, 

 Thompson ; Carnlough co. Antrim, and bog-holes co. Wicklow, Mr. 

 D. Moore. Dolgelley ; Tavistock and Trentishoe, Devonshire ; Pen- 

 zance. 



Brownish when recent, whitish brown when diy. 



Filaments elongated, attenuated, very fragile, separating into 

 single frustules almost immediately after being gathered. At 

 first the frustules are as broad as long, but when mature they are 

 generally two or three (occasionally five or six) times longer than 

 broad ; the puncta at the ends are more minute than in F. pecti- 

 nalis and F. confervoides, and along the lateral margins are di- 

 stinct rather distant puncta, which are the terminations of the 

 lateral striae. The lateral surfaces are broad, with well-marked 

 transverse striae. In the smaller frustules there are only two or 

 three striae situated near the middle; they afterwards become 



