292 Proceedings of the Royal Irish Academy. 
Diatoma elongatum, (Agardh.) Fresh water. 
Frustules on front view greatly inflexed ; on side view linear, nar- 
row, with capitate and expanded ends; valves not so much arched asin 
the preceding species, so that the coste 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. Kutz. Bac., p. 48, TWxvim., fic. 18. Rab. 
Sussw..Diat., p35, 7. i, ie 1. Wim, Min, ed), Olam. aia, 
PI xls fg. O11. Talis Pritct., po. 279, ei Sy., He de le ihe 
fig. 169. Heiberg, De Danske Diat., p. 57. Castracane, Catalogo di 
Diat., raccolte nell Val. Intrasca, p. 15. Cleve, Om Svenska och 
Norska Diat., p. 219.—Diatoma tenue var. elongatum, Grunow, Ver- 
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.) Fresh water. 
Frustules small ; on front view regularly quadrangular ; ends of the 
cost appearing like a fine line of puncta; on side view broadly ellipti- 
cal ; ends rounded ; costée 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 quadrangular; 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. For 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 features are perfectly distinctive. The valve 
of D. vulgare is greatly arched, so that the ends of the coste occupy a 
large portion of the front view; whereas in D. tenue, the valve is 
flat, and the ends of the cost, on the front view, are barely noticeable. 
Kiitz. Bac., p. 48, T. xvii., fig. 9,10. Rab. Stissw. Diat., p. 39, 
T. ii., fig. 5. Ralfs, in Pritch., p. 779. Grunow, Verhand der K. K. 
Zool. Bot. Gesel., Band xii., 1862, p. 362.—Diatoma elongatum, variety 
¥. Wit. Sul... De, Volru, ps 20,.Ee xicme. old 2. 
