340 Proceedings of the Royal Irish Academy. 



As to the second, Pfitzer is at variance witli most oilier authors who 

 have regarded the forms included under the genus Pinnularia as per- 

 fectly symmetrical, and to me they have ever appeared just as symme- 

 trical as those of Navicula. The third characteristic is that which is 

 most ■worthy of notice, but the forms in which the peculiarity has been 

 observed are comparatively few. So that we are not as yet in a posi- 

 tion to regard it as satisfactorily established. Eor myself I have long 

 since regarded the distinction between Navicula and Pinnularia as unsa- 

 tisfactory, and have felt obliged to abandon it in consequence of having 

 observed forms in which the costate character of the Pinnularise is- 

 combined with the moniliform striae of the Naviculag. In consequence 

 of this there is no alternative but the abandonment of the genus Pin- 

 nularia, or the adoption of a new genus to receive these forms in 

 which the characteristics of ISTavicula and Pinnularia are combined. The- 

 former appears the more satisfactory course, which I have accordingly 

 pursued. The forms belonging to this genus are now so very nume- 

 rous some more satisfactory grouping of them than that of Smith and 

 Ralfs, founded on the outline of the valves, is necessary. Grunow 

 has done much towards supplying this desideratum, and, if I hava 

 succeeded in effecting an improved arrangement, I am indebted to the 

 hints supplied by that distinguished naturalist. 



Conj ugation has been observed in some species of Navicula. Twa 

 mother cells produce two sporangial cells or auxospores, as Pfitzer 

 designates them, which latter are found to lie in a position parallel to 

 that of the former. 



(a.) Noliles. 



Strice strongly costate, not extending to the median line, hut leaving 

 a hroad, smooth, longitudinal middle space, which is expanded around th& 

 central nodule, and occasionally extending to the margin. 



Navicida nolilis, (Ehr.) Fresh water. 



Yalve large, varying in length from -012 to -015 ; oblong, inflated 

 both at the middle and ends; costte broad, converging in the middle^ 

 and slightly radiate towards the ends ; longitudinal free median space 

 expanded greatly at the centre and ends. (Plate 30, fig. 1.) 



Kutz. Bac, p. 98, T. iv., fig. 24. Ralfs, in Pritch., p. 895. Gru- 

 now, Yerhand. der K.K. Zool. Bot. Gesel., Band x., 1860, p. 515. Cleve, 

 Om Svenska och JSforska Diat., p. 223. — Pinnularia nobilis, Ehr. 

 Proc. Berl. Acad., 1840. Wm. Sm., B.D., Yol. i., p. 54, PI. xvii., fig. 

 161. Rab. Siissw. Diat., p. 44, T. vi., fig, 2 ; Eab. El. Eur., Alg., sect. 

 1, p. 209. 



Bantry, Co. Cork. Featherbed Mountain, Co. Dublin. Lugna- 

 quilla, Co. Wicklow. Lough Mourne deposit, Co. Antrim. Dromore 

 Bub-peat deposit, Co. Down. Eiver Bann, at Coleraine, Co. Derry. 

 Drumoughty Lough, near Kenmare, Co. Kerry. 



