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 Brightwell, 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 Kitzingiana, (Thwaites.) Fresh water. 
Frustules undulate ; strize delicate, marginal, scarcely one half of 
the radius in length; the central portion of the valve unstriate. (Pl. 
26, fig. 10. 
Smith deena this species to fresh or brackish water, and with 
this opinion Rabenhorst concurs. Walker-Arnott 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. 
Kiitzingiana, 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 influences, 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 Wm. Smith accounts for the ap- 
pearance, and his opinion on the subject is supported by that of 
Heiberg. 
Wm. Sm., B.D.) Vol-i., p. 27) Plo v.; ne: 4h eo Rabeny > Marg 
p. 82.—Orthosira Kiitzingiana, Heib. De Danske Diat., p. 31. 
Stream, Crossdoney, Co. Cavan. Stream near Larne, Co. Antrim. 
Tacumshane, Co. Wexford. Ditch on banks of River Liffey, Co. 
Kildare, near Ballymore Eustace. Tarbert, Co. Kerry. Ditch, Kal- 
cool, Co. Wicklow. 
Cyclotella Meneghiniana, (Kiitz.) Fresh water. 
Frustules not undulate on the front view; strie on the valves 
much coarser than in the former species, more distant, and consider- 
ably longer. (PI. 26, fig. 11.) 
Walker-Arnott regards this species as identical with C. rectangula, 
De Bréb, and the greater weight attaches to his opinion as it was 
adopted after examination of specimens ‘‘from De Brébisson himself, 
