256 Proceedings of the Royal Irish Academy. 



on the other. Yet his figures, however obscure in minute details of 

 structure, distinctly mark the separation of the valves into two well- 

 defined concentric parts. 



Walker-Arnott confounded the distinction between this genus 

 and Orthosira, when he identified Cyclotella dallasiana with the Cyclo- 

 tella radiata of Erightwell, which should rather be included in Or- 

 thosira. There is great confusion as to the synonomy of the several 

 species; nor is this surprising, as the earlier descriptions and figures 

 are by no means satisfactory ; and, in order to avoid consequent per- 

 plexity, I consider there is no more satisfactory plan than to follow 

 the line marked out by the figures of Wm. Smith, which are so distinct 

 as to be easily recognised. 



Cyclotella Kut%ingiana, (Thwaites.) Fresh water. 



Frustules undulate ; striae delicate, marginal, scarcely one half of 

 the radius in length ; the central portion of the valve unstriate. (PL 

 26, fig. 10.) 



Smith attributes this species to fresh or brackish water, and with 

 this opinion Rabenhorst concurs. AValker-Amott alleges he has 

 never seen the true species from fresh water. Cleve makes it an 

 essentially brackish water species, and Heiberg on the contrary, a 

 fresh water form. This difference of opinion may arise from mistake 

 as to identity ; but speaking of the form described by Smith as C. 

 Kutzingiana, I have to say that though I have sometimes found it in 

 water slightly brackish, it has been commonly discovered by me in 

 localities far remote from marine infiuences, so that I think it is to be- 

 considered an essentially fresh water species. 



The slightly undulate outline which this species presents on the 

 front view may possibly: arise from the sinking in of the valve in the 

 centre, and the consequent projection of the outline of the dip upon 

 the plane of observation. Thus "\Ym. Smith accounts for the ap- 

 pearance, and his opinion on the subject 4s supported by that of 

 Heiberg. 



Wm. Sm., B.D., Yol. i., p. 27, PL v., fig. 47. Eaben. PL Eur., 

 p. 32. — Orthosira Kiitzingiana, Heib. De Danske Diat., p. SI. 



Stream, Crossdoney, Co. Cavan. Stream near Larne, Co. Antrim. 

 Tacumshane, Co. "VYexford. Ditch on banks of Eiver Liffey, Co. 

 Kildare, near Ballymore Eustace. Tarbert, Co. Kerry. Ditch, Kil- 

 cool, Co. Wicklow. 



Cyclotella Meneghiniana, (Kiitz.) Presh water. 



Prvistules not undulate on the front view ; striae on the valves, 

 much coarser than in the former species, more distant, and consider- 

 ably longer. (PL 26, fig. IL) 



Walker- Arnott regards this species as identical Avith C. rectangula, 

 De Breb, and the greater weight attaches to his opinion as it was- 

 adopted after examination of specimens "from De Brebisson himseK^ 



