O'Meara — Rej)0)'t on the Irish Diatomacece. 255 



Orthosira Roesemia, (Eab.) Presh water. 



Inner surface of cell sub-sph.erical ; frustule sulcate on either side 

 of suture ; spines at the junction surfaces very distinct and long ; cir- 

 clet of puncta parallel with suture absent. Striae on side view radiate, 

 distinct, with three large puncta placed triangularly at the centre. 

 On front view striae finely punctate, and parallel with suture. 

 (PL 26, fig. 9.) 



Smith, in 1856, describes this species as new, under the name of 

 Orthosira spinosa. "Wm. Sm., B. D., Vol. ii., p. 62, PI. Ixii., fig. 

 386. But it had been already described by Rabenhorst, Siissw. Diat., 

 p. 13, T. X., fig. 5, in 1853, as Melosira Roeseana, and with sufiicient 

 accuracy, both as respects the figure and the description, as to render 

 identification certain. Melosira Roeseana, Ralfs, in Pritch., p. 818, 

 PL v., fig. 67. 



Killakee, County Dublin. Ulster Canal, near Poyntzpass. Lough 

 Neagh, near Lurgan, County Armagh. Ditch at side of Royal Canal, 

 near Kilcock, County Kildare. 



Genus V. Cyclotella, Kiitz. 



Prustules normally single, narrow ; sometimes slightly waved on 

 the front view ; on the side view having the valves more or less dis- 

 tinctly divided into two concentric portions. 



It is extremely diificult to define this genus by words so precisely as 

 to distinguish it with certainty from others nearly allied ; yet still 

 the forms included within it constitute a tolerably distinct group. So 

 much so, that almost all authors haA'e agreed to mark their peculiarity 

 by a distictive generic name. 



Heiberg and Cleve have included the several species under the 

 genus Orthosira, with which they are closely allied ; but I consider 

 them entitled to stand by themselves, not only on account of their 

 different modes of growth, but also on account of the distinctive cha- 

 racters of their sporangia. 



It would appear at first view that the generic name Discoplea 

 should, on account of its priority, be preferred to the more recent name 

 •of Cyclotella. As Ehrenberg has given no verbal diagnosis of his genus 

 Discoplea, we have no means of ascertaining its characteristics, other- 

 wise than by the figures, and in these no sufficiently distinctive feature 

 is discernible. JSTot only are forms that seem to belong to different 

 species included under the same specific name, but more than this, 

 species belonging to Orthosira, on the one hand, and more closely re- 

 sembling Coscinodiscus, on the other, are included in the genus Disco- 

 plea. Kiitzing's diagnosis of his genus Cyclotella, although sufficient 

 to distingush it from Orthosira, on the one hand, is not clear enough, so 

 far as words are concerned, to prevent confusion with Coscinodiscus, 



