BRITISH DIATOMACEiE. 29 



founded. Its distinctive characters are an abbreviated stipes, which generally 

 assumes the form of a mere cushion, and never reaches half the length of the 

 frustule, and elliptical valves with rounded, and not attenuate, extremities. It 

 is a much rarer plant than either of the former ; or, being more incon- 

 spicuous, has been more frequently overlooked. A remarkable instance of 

 the abnormal development adverted to under Meridion circulare has presented 

 itself in the present species, an example of which is figured in Plate XXXVIII. 

 302*. Here, the formation of a cell interior to the original one has proceeded 

 through several successive stages, and the result is a compound frustule, con- 

 sisting of the mother-cell and a number of included cells, each successive 

 development being embraced by the others previously formed. 



Plate XXXVII. 302. 



4. Achnanthes exilis, Kiltz. Filaments from 2 to 50 frustules ; 

 V. linear-lanceolate, with obtuse extremities, occasionally some- 

 what produced; striae very faint. Length of frustule -0006" to 

 •0018". Stipes very variable in length, subgregarious or solitary, 

 v.v. 



Kiitz. Bacill. xxi. 4. ad specim. quae dedit am. De Brebisson. Ralfs, Ann. 

 vol. 13. pi. xiv. 12. ad specim. quae dedit cl. Auctor. Hass. Alg. c. 5. 

 Raben. Siissw. Diat. viii. 1 . Achnanthes minutissima, Ralfs, Ann. I. c. 

 fig. 1 1. Hass. Alg. /. c. fig. 4. ad specim. authen. in Herb. Jenn. 



Fresh water. Near Lewes, Sept. 1852, March 1853. Parasitic on the 

 stipes of Gomphonema geminatum, Galway, July 1853 ; and Grassmere, Aug. 



1853, W. Sm. Parasitic on the stipes of Gomphonema naviculoides in the 

 tank of the Victoria regia, Edinburgh Botanic Gardens, Sept. 1853, Dr. 

 Greville. Monkland Canal near Glasgow, Oct. 1853. Mr. R.Hennedg. Salt- 

 coats, Ayrshire, Jan. 1854, Dr. Arnott. On Gomphonema geminatum, Dol- 

 gelly, Mr. Ralfs. Wray, Lancashire, July 1850, Mr. Geo. Smith. Cheshunt, 

 Mr. Hassall ; communicated by Mr. Jenner. (Well near Marseilles, May 



1854, W.Sm.) 



I am unable to find constant characters in the numerous British specimens 

 in my possession by which to constitute two species of the above, and feel 

 constrained to regard A. minutissima of Ralfs as a mere state or variety of the 

 present plant. It will be seen, by a reference to the figures given by Mr. 

 West, how sportive in habit of growth and length of filament are the 

 specimens with which I furnished him, and yet each extreme may not unfre- 

 quently be found in close neighbourhood on the plant which serves as their 

 support. 



The length of the stipes, upon which Mr. Ralfs seems mainly to rely, I 

 cannot accept as a specific character in the present species, as it seems to me 

 to be chiefly dependent upon the stage of growth. Thus Mr. Ralfs says, that 

 in A. minutissima " the stipes is not longer than the frustule," while in 

 A. exilis " the stipes exceeds the frustule in length." Now, in the specimens 



