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 curved: 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 surfaces 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 striae along the lateral 

 margins of the front view. 



Usually the frustules are nearly colourless, 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 drying. 



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

 cept Ach, minutissimuj by its freshwater habitat, its slender frus- 

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

 it differs in its elongated stipes and more acute lateral surfaces. 



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

 His, 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 surfaces 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 : b, frustule deprived of its colour- 

 ing matter ; c, lateral view. 



Analysis. 



, ( Fluvatile, striae wanting or indistinct 2 



' \ Marine or submarine, lateral striae evident >, 3 



{Stipes much longer than the frustule ; lateral surfaces 

 subacute exilis. 

 Stipes not longer than the frustule ; lateral surfaees ob- 

 tuse minutissima, 



2 r Stipes longer than the frustule longipes. 



' \ Stipes shorter than the frustule 4 



r Lateral view lanceolate, with acute ends; lateral striae 



. I strongly marked, dotted brevipes. 



j Lateral view elliptic, with obtuse ends; striae apparently 



L not dotted subsessilis. 



N.B. Figure 6 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: 6, a frustule deprived of its colouring matter; 

 c, lateral view. 



