318 Proceedings of the Royal Irish Academy. 
ends of the diaphragms alternate ; valyes much expanded in middle ; 
strongly costate ; coste pervious. 
Tetracyclus lacustris, (Ralfs.) Fresh water. 
Middle expansion of the valve rounded. (Plate 29, fig. 4.) 
Ralis, Ann. Nat. Hist., Vol. xu., 1843, Pl. u., fig. 2. Kutz. Bae., 
p. 127, 7. xx. fig. 70. Rab: Siisaw: Diat. p. 08," 2. 1m. ne oe 
Wm. Sm., B. D., Vol. ., p.38,"Pl xxxx, fes3s08. Ralis in Prten 
p. 806, Pl. vi., fig. 10, and Pl. x1., fig. 24, 25. Grunow, Verhand 
der K. K. Zool. Bot. Gesel., Band xii., 1862, p. 412. Cleve, Om 
Svenska och Norska Diat., p. 222. 
River Erne, Crossdoney, Co. Cavan. Lake near Castlewellan, Co. 
Down. River Bann, Verner’s Bridge, Co. Armagh. Tonabrick Moun- 
tain, Co. Cork; Wm. Smith. 
Tetracyclus emarginatus, (Ehr.) Fresh water. 
‘¢ Valves constricted towards the extremities, which are rounded 
and sub-apiculate ; inflections deeply notched or emarginate ; otherwise 
like the last species.” —Wm. Smith. 
Wm. Sm. B, D., Vol. ii., p. 38. Ralfs, in Pritch., p. 806. Grunow, 
Verhand. der K. K. Zool. Bot. Gesel., Band xii., 1862, p. 412. Rab. 
Fl. Eur., sect. 1, p. 302. Cleve, Om Svenska och Norska Diat., p. 222. 
Smith supposes this species identical with Biblarium emarginatum, 
Ehr. Mic. T. xxxiu. 2, fig. 6. On his authority I attribute the 
species to Ehrenberg. 
Gap of Dunloe, Killarney. Wm. Smith. This species is extremely 
rare, not a single specimen haying ever come under my notice from 
any locality in Ireland. 
Genus lV. Ruasponema, Kitz. 
Stipes short; diaphragms numerous, on the external margin 
strongly costate, broad; extremity of the valves unstriate. 
Smith alleges that in this genus the valves have a median line, a 
statement which does not appear to be sustained by the facts of the 
case. 
Rhabdonema arcuatum, (Lyngb.) Marine. 
Frustules short; on side view broadly elliptical; costate, with 
moniliform striz interposed between the cost; striz pervious; dia- 
phragms numerous, parallel, with a single foramen. (Pl. 29, fig. 5.) 
Kitz. Bac., p. 126, T. xviii., fig. 6, who states that the species is 
