492 Mr. J. Ralfs on the British species o/Aclinanthes. 



striae, and the lines separating it from the lateral portions are less 

 distinct than in the two preceding species. I have not been able 

 to ascertain whether the striae are dotted. The stipes is short and 

 thick, as in Achnanthes brevipes. 



This species comes very close to Ach. brevipes, but is a smaller 

 plant ; the frustules are less bent, the lateral surfaces are not so 

 prominent in the front view, the striae are less distinct, the cen- 

 tral portion has very obscure markings or none at all, and espe- 

 cially the lateral view is elliptic and obtuse at the ends. 



The endochrome is pale brown, and generally collected into 

 two faint, roundish central spots. There is frequently a small 

 air-hke globule near each end of the recent frustule, which dis- 

 appears when it is dried. 



Plate XIV. fig. 10. Achnanthes suhsessilis : b, frustules deprived of their 

 colouring matter ; c, superior lateral view ; d, inferior lateral view. 



4. A. minutissima, Ktz. Frustules slender; lateral surfaces obtuse, 

 strise apparently wanting ; stipes not longer than the frustule. 

 Ktz. Syn. Diatom, p. 578. f. 54; Ktz. Alg. aq. dulc. no. 75 !; 

 Ehr. Infus. p. 228. pi. 20. f. 5 ; Pritch. Infus. p. 233. 



In freshwater ponds, &c., parasitic on other algae. Newtimber, 

 Sussex, Mr. Borrer ; Shoreham, Kent, Mr. Jenner ; Cheshunt, Mr. 

 Hassall; Oswestry, Shropshire, Rev. T. Salwey ; common about Pen- 

 zance on Bulbochcete setigera, J. R. ; near Bristol, Mr. Thwaites. 



I believe that this species is very common, but generally over- 

 looked from its minuteness. 



The fronds are scattered, extremely minute, and rarely of more 

 than two frustules. The frustules are two to six times as long as 

 broad, slightly curved and colourless. The stipes is not longer 

 than a frustule, and the punctum on the lower margin is very 

 minute. 



Before I had the opportunity, through Mr. Borrer's kindness, 

 of comparing our plant with the specimen in Kutzing's ' Alg. aq. 

 dulc.,' I considered it a variety of the following species ; and I am 

 still uncertain whether they should be separated, although the 

 plant above described is undoubtedly Kutzing's Achnanthes mi- 

 nutissima. 



Plate XIV. fig. 11. Achnanthes minutissima : b, frustule deprived of its 

 colouring matter ; c, lateral view. 



5. A. exilis} Ktz. Frustules slender; lateral surfaces subacute; 

 striae indistinct or wanting ; stipes longer than the frustule. Ktz. 

 Syn. Diatom, p. 578. f. 53 ; Ktz. Alg. aq. dulc. no. 12 ; Ehr. Infus. 

 p. 228. pi. 20. f. 5 ; Pritch. Infus. p. 233. 



Parasitic on Gomphonema geminatum in several streams near Dol- 

 gelley. 



It is dark reddish-brown when recent, and turns greenish in 

 drying. 



This species is very minute : the frustules in a frond vary from 



