BRITISH DIATOMACEiB. 87 



valve is ridged or waved. The genus Actinoptycus of Ehrenberg is only 

 distinguished from Aetinocyclus by the presence of internal septa : these do 

 not exist in any specimens I have seen, and, I am disposed to believe, are in no 

 case present in frustules of this kind. 



17. Triceratium exiguum, n. sp. Cellules circular; angles pro- 

 duced into linear and truncated processes. Distance between the 

 angles "0005". v.v. 



Fresh water. Ormesby, Norfolk, Aug. 1853, Mr. Bridgman. 



This beautiful little form is remarkable as being the only freshwater species 

 of the genus. 



18. Triceratium striolatum, Ehr. Cellules circular, distinct, in 

 subradiate series ; margin of valve convex ; angles produced into 

 short subconical truncate processes, v.s. 



Ehr. in Kiitz. Bacill. xvii. 10. Roper, Mic. Trans, vol. 2. pi. vi. 3. 



Marine. "Thames mud," Mr. Roper. 



The species described in the present work, Vol. i. p. 27, is probably T. un- 

 dulatum, Ehr. 



19. Triceratium armatum, Roper. Cellules circular, irregularly 

 arranged ; margin of valve concave ; angles produced, subcapi- 

 tate. v.s. 



Roper, Mic. Journ. vol. ii. p. 283. fig. 1. 



Marine. Milford Haven, July 1854, Mr. Okeden. 



20. Cyclotella punctata, n. sp. F. V. with undulations; valves de- 

 licately punctate or cellulate; cellules radiate. Diameter *0008" to 

 •0015". v.s. 



Fresh' water. Near Wisbeach, Dec. 1853, Mr. S. Smith. 



21. Cyclotella Dallasiana, n. sp. Margin of valve costate; centre 

 cellulate ; cellules irregular. Length of costas '0002". Diameter 

 of valve -0022". v.s. 



Marine ? Medway, Mr. Dallas. 



The single specimen of this species in balsam, communicated by Dr. Gre- 

 ville, leaves much in uncertainty respecting its true character. 



