292 Proceedings of the Royal Irish Academy. 



Diatoma elongatum, (Agardh.) Fresh, water. 



Prustules on front view greatly inflexed ; on side view linear, nar- 

 row, with capitate and expanded ends ; valves not so much, arched as in 

 the preceding species, so that the costae appear, on front view, as a row 

 of puncta. 



There is considerable diversity in the outline of various forms of 

 this species ; in some the capitate ends are not so much expanded as in 

 others. In some the margin on side view, instead of being perfectly 

 straight, is slightly expanded towards the middle. The species might, 

 in some cases, be confounded with the preceding ; but the characters 

 above given will serve to distinguish between them. 



Agardh, Syst., p. 4. Kiitz. Bac, p. 48, T. xviii., fig. 18. Kab. 

 Siissw. Diat., p. 35, T. ii., fig. 1. Wm. Sm., B. D., Yol. ii., p. 40, 

 PI. xl., fig. 311. Ealfs, in Pritch., p. 779, PL iv., fig. 14, PI. ix., 

 fig. 169. Heiberg, De Danske Diat., p. 57. Castracane, Catalogo di 

 Diat., raccolte nell Yal. Intrasca, p. 15. Cleve, Om Svenska och. 

 ITorska Diat., p. 219. — Diatoma tenue var. elongatum, Grunow, Yer- 

 hand der K. K. Zool. Bot. Gesel., Band xii., 1862, p. 363. 



Dundrum, Co. Dublin. ^Newcastle, Co. "Wicklow. (Cork harbour. 

 Belfast, Wm. Sm.) 



Diatoma tenue, (Kiitz.) Freshwater. 



Prustules small ; on front view regularly quadrangular ; ends of the 

 costae appearing like a fijie line of puncta ; on side view broadly ellipti- 

 cal ; ends rounded ; costse fine. 



Smith makes this species a variety of Diatoma elongatum. Gru- 

 now regards it as the representative of a species of which. Diatoma 

 elongatum is a variety. A careful examination of the form, I think, 

 will lead to the conclusion that it deserves to rank as a distinct species. 

 In general appearance, in outline, both on front and side view, it differs 

 from Diatoma elongatum ; on front view D. elongatum is not regularly 

 quadrangular, but somewhat inflexed at the sides, the ends being 

 broader than the middle ; whereas in D. tenue, the front view nor- 

 mally is perfectly quadi^angular ; on the side view, D. elongatum is 

 more or less distinctly capitate, the ends being broader than the 

 middle ; the sides are usually straight and parallel ; in D. tenue the 

 side view in outline is broadly elliptical ; the ends narrowed and 

 rounded. Por these reasons I consider D. tenue is obviously distinct 

 from D. elongatum. It might more likely be considered a variety of 

 Diatoma vulgare, but its featiu'es are perfectly distinctive. The valve 

 of D. vulgare is greatly arched, so that the ends of the costse occupy a 

 large portion of the front view ; whereas in D. tenue, the valve is 

 flat, and the ends of the costse, on the front view, are barely noticeable. 



Kutz. Bac, p. 48, T. xvii., fig. 9, 10. Rab. Siissw. Diat., p. 35, 

 T. ii., fig. 5. Ealfs, in Pritch., p. 779. Grunow, Yerhand der K. K. 

 Zool. Bot. Gesel., Band xii., 1862, p. 362. — Diatoma elongatum, variety 

 y. Wm. Sm., B. D., Yol. ii., p. 40, PI. xH. fig. 311 y. ' 



