Mr. J. Ralfs on the British species of Achnanthes. 493 



one or two to nine, and are occasionally even more numerous ; they 

 are slender, three to six times as long as broad, and cm-ved: the up- 

 per margin is convex in the middle and concave on each side ; the 

 lower margin, on the contrary, is convex at the ends and concave 

 in the centre with a minute punctum, which is scarcely discern- 

 ible until the endochrome is removed. The stipes is slender, 

 often curved, and from three to five times as long as a frustule; 

 two or more are often connected together at the base. The la- 

 teral sm"faces are flat, narrow, lanceolate, with subacute ends. In 

 general there is no appearance of striae, but I believe this depends 

 upon their extreme minuteness ; for in one or two instances, after 

 the endochrome had been destroyed, I distinctly observed in some 

 of the larger frustules the terminations of strise along the lateral 

 margins of the front view. 



Usually the frustules are nearly colom*less, whilst the endo- 

 chrome forms a very pale, central, irregular patch, which occasion- 

 ally indeed is of a dark brown. At each end of this patch there 

 is almost always, in the recent state, a colourless air -like globule 

 which disappears in di'ying. 



This species is easily distinguished from all the preceding, ex- 

 cept Ach. minutissima, by its freshwater habitat, its slender frus- 

 tules, and the apparent absence of striae. From Ach. minutissima 

 it chffers in its elongated stipes and more acute lateral sm^faces. 



Although I have compared our plant with Kutzing^s Ach. ex- 

 ilis, I am not certain that it is the same species. It agrees with 

 Kutzing's specimen in its crowded habit and elongated stipes, but 

 its frustules are much smaller and its lateral sm-faces less acute, 

 in both which respects it is intermediate between Ach. minutissima 

 and Kutzing's specimen of Ach. exilis. 



Plate XIV. fig. 12. Achnanthes exilis : h, frustule deprived of its colour- 

 ing matter ; c, lateral view. 



Analysis. 



, I Fluvatile, strise wanting or indistinct 2 



* \ Marine or submarine, lateral stri« evident 3 



i Stipes much longer than the frustule ; lateral surfaces 

 subacute exilis. 

 Stipes not longer than the frustule ; lateral surfaces ob- 

 tuse minutissima. 



o r Stipes longer than the frustule lo7igipes, 



' (Stipes shorter than the frustule 4 



r Lateral view lanceolate, with acute ends ; lateral striae 



. J strongly marked, dotted hrevipes. 



'I Lateral view elliptic, with obtuse ends; striae apparently 

 L not dotted suhsessilis. 



N.B. Figure G of Plate XIV. is a representation oi Striatella unipunctata, 

 which was described at p. 456 of Vol. XL, but the figure of which was then 

 omitted from want of space : b, a frustule deprived of its colouring matter; 

 c, lateral view. 



