BRITISH DIATOMACEiE. 17 



Himantidium tetraodon, Breb. in Kutz. Sp. Alg. p. 10. ad spec, quae misit 

 amic. De Brebisson, Jan. 1853. 



Fresh water. Ashdown Forest, Sussex, Aug. 1850, W. Sm. Curwen 

 Hill, Lancashire, Mr. G. Smith, June 1851. Quernmore, Lancashire, April 

 1852, Mr. Johnson. Mountains in Aherdeenshire, Dr. Dickie. Dolgelly, 

 Mr. Raffs. Dolgelly Earth. Peterhead Deposit. (Manchester, Mass., com- 

 municated by Prof. Williamson.) 



Plate II. 19. 



7. Eunotia Diadema, Ehr. V. with six dorsal ridges ; striae .30 in 

 •001". Length -0016" to -0024". v.s. 



Ehr. Inf. xxi. 27. Kiitz. Bacill. v. 28. 

 Fresh water. Dolgelly Earth. 

 Plate II. 20. 



Genus 3. CYMBELLA, Ag. 



Frustules free, cymbiform ; valves striated, with a submedian line and 

 central and terminal nodules. 



Many frustules of Cocconemata detached from their stipes bear so 

 close a resemblance to those of the Cymbellce, that a discrimination 

 can hardly be relied upon which depends solely upon the observation 

 of prepared specimens. In a living state, the presence of a stipes at 

 once removes such frustules from the present genus. Cymbella fre- 

 quently forms a more or less distinct stratum, the frustules being re- 

 tained in their position, even when exposed to a rapid current, by the 

 presence of a delicate pellicle of mucus. 



1. Cymbella Ehrenbergii, Kiitz. V. elliptical-lanceolate, extre- 

 mities slightly produced, obtuse; striae distinct, 15 in -001". 

 Length -0028" to -0054". v,s. 



Kiitz. Bacill. vi. 11. NaviculaincBqualis, Ehr. Inf. xiii. 18. Prit. Anim. iii. 

 154. 



Freshwater. Living, in Lough Mourne, Sept. 1849. Peterhead Deposit. 

 Lough Mourne Deposit. PremnayPeat. Marl, Co. Down. (Near Manchester, 

 Mass., communicated by Prof Williamson. Very abundant in fossil earth, 

 San Fiore, Italy.) 



Plate II. 21. 



