400 Proceedings of the Royal Irish Academy. 



tinct species, and named IS'avictila donkinii, the name wliich I had 

 given to it in my list before the Atlas had come under my notice. 

 The form here described agrees precisely with Gregory's description of 

 I^avicula musca, which may readily be distinguished from jN^avicula 

 donkinii, by its much deeper constriction, and the shai'p outline at the 

 ends, in consequence of which it resembles the abdomen of a fly ;_ 

 the strise, too, in this are punctate, while in the other they arc 

 costate. 



Piles of wooden bridge on DoUymount Strand, Co. Dublin. 



Nmicula interrwpta, (Kiitzing). Marine. 



Valve deeply constricted ; lobes suborbicular ; median compart- 

 ment broad, greatly inflesed at the ends, considerably constricted in 

 the middle ; compartments on either side very narrow, bilunate,. 

 unstriate ; marginal band of stride very narrow in the middle, where 

 the strise seem to fail, but tolerably wide in the middle of the lobes ; 

 stride costate, nearly parallel in the middle, radiate towards the ends ; 

 length -0023, breadth, -0010 ; at constriction -0007. (Plate 33, fig. 26.) 



Kiitz. Bac, p. 100, T. xxis., fig. 93. Ealfs, in Pritch., p. 894. 

 Eab. PI. Eur. Alg., sect, i., p. 205. Donkin, ^''. H. Brit. Diat., p. 47, 

 PI. vii., fig. 2. Schmidt, Atlas, T. xii., fig. 2. — jSTavicula didyma, 

 Wm. Sm., P. D., Yol. i., p. 53, PL xvii., fig. 154a. 



Ballysodare, Co. Sligo. Lough Gill, Co. Kerry. Arran Islands ;■ 

 Stomachs of Ascidians, Roundstone Bay ; Stomachs of Ascidians^. 

 Broaclhaven Bay, Co. Galway. Seaweeds, coast of Co. Clare. 



Nmicula apis, (Ehr.) Marine. 



Yalve deeply constricted, ends narrowed and rounded ; median 

 compartment broad, with well-defined boundary lines, slightly inflexed 

 at ends, slightly constricted in the middle ; compartments at either 

 side unstriate, narrow, tapering to a point at the ends ; marginal 

 band of strife narrow in the middle, increasing considerably, and then 

 narrowing towards the ends ; striae in the middle apparently costate, 

 convergent towards the ends, radiate, and having the apjoearance more 

 of fine costse interrupted by close longitudinal sulci, than of being 

 moniliform ; length -0038, breadth '0011 ; breadth at the constric- 

 tion -0008. (Plate 33, fig. 27.) 



There is great difiiculty in identifying the species so named, and 

 with some hesitation have I come to my conclusion on the subject. 

 Kiitzing' s figure of Navicula apis is shorter and stouter than the pre- 

 sent, and the striae are so indistinct as to furnish no help. Donkin's 

 figure in outline is precisely the same as in the form under considera- 

 tion; the striae, however, are represented as more decidedly punctate, 

 and the compartments on either side of the median one are distinctly 

 striate. In the present case, there is sometimes an appearance of strite 



