O’Mrara—Report on the Irish Diatomacee. 285 
Genus II. Denrticuza, Kiitz. 
Frustules united in parallel filaments; on front view regularly quad- 
rangular, on side view narrow, elliptical, costate, costae not pervious. 
Kiitzing’s distinction between this genus and Odontidium is not 
very obvious. Smith entertains considerable doubt as to the propriety 
of separating them, but distinguishes them by the relative length of 
the filaments. Those whose frustules form short filaments, he attri- 
butes to Denticula, while those forming filaments of considerable 
length constitute the genus Odontidium. Ralfs retains the two genera, 
interposing that of Plagiogramma between them, and remarks that, in 
the valves of Denticula, fine strize are interposed between the cost, 
this peculiarity not being noticeable in the valves of the several species 
of Odontidium. Rabenhorst also retains the two genera, and inter- 
poses the genus Gomphogramma between them, his distinguishing 
character being the same as that on which Smith relies. Grunow 
relegates the genus Denticula to the group Nitzschiee, separating 
from it Denticula obtusa, Kiitz., which he includes under Fragilariee ; 
while Heiberg, who appears to be followed by Cleve, drops both 
genera, referring Odontidium parasiticum, Wm. 8m., to the genus 
Fragilaria, and Odontidium mutabile, Wm. Sm., to the genus Dia- 
toma. 
The distinctive characters of the genera, as here defined, rest on 
the fact that, in Denticula, the costs are interrupted by a broader or 
narrower intermediate space, while the cost in Odontidium are per- 
vious. 
Denticula obtusa, (Kiitz.) Fresh water. 
Filaments short’; on side view narrow; elliptical, coste marginal, 
with fine pervious strie interposed. 
Kiitz. Bac., p. 44, T. xvii., fig. 14. Rab. Siissw. Diat., p. 33, T.i., 
ngs. Wa, Do, Vol. 1-,p..19, Pl. xxxiy., fic. 292. Ralis. m 
Pritch,, p-, 41.0. 
River Dodder, Basin of Grand Canal, Co. Dublin. 
Denticula mutabilis, (Wm. Sm.) Fresh water. 
Frustules varying greatly in size, generally forming long filaments ; 
on side view nearly oval, and sometimes narrow, elliptical; costae 
broadly marginal, without interstitial strie. (Pl. 27, fig. 17.) 
Odontidium mutabile, Wm. 8m., B. D., Vol. ii., p. 17, Pl. xxxiv., 
fig. 290. Grunow, Verhand der K. K. Zool. Bot. Gesel., Band xii., 
1862, p. 369.—Fragilaria mutabilis, Rab. Fl. Eur., sect. 1, p. 118. 
Diatoma mutabile, Heiberg, De Danske Diat., p. 58. Cleve, Om 
Svenska och Norska Diat., p. 219.—Dimeregramma mutabile, Ralfs, in 
Pritch., p. 790. 
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