O'Meara — Re2:)ort on the Irish Diatomacece. 285 



Genus II. Dentictjla, Kiitz. 



Frustules uiiited in parallel filaments ; on front yie-w regularly quad- 

 rangular, on side view narrow, elliptical, costate, costse not peiTious. 



Ktitzing's distinction between this genus and Odontidiuni 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, Ealf s retains the two genera, 

 interposing that of Plagiogramma between them, and remarks that, in 

 the valves of Denticula, fine striae are interposed between the costae, 

 this peculiarity not being noticeable in the valves of the several species 

 of Odontidium. Eabenhorst 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 Mtzschiese, separating 

 from it Denticula obtusa, Kvitz., which he includes under rragilariese ; 

 while Heiberg, who appears to be followed by Cleve, drops both 

 genera, referring Odontidium parasiticum, "Wm. Sm., to the genus 

 Pragilaria, 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 costae are interrupted by a broader or 

 narrower intermediate space, while the costae in Odontidium are per- 

 vious. 



Denticula oitusa, (Kiitz.) Fresh water. 



Filaments short ; on side view narrow ; elliptical, costae marginal, 

 with fine pervious stride interposed. 



Kiitz. Bac, p. 44, T. xvii., fig. 14. Eab. Siissw. Diat., p. 33, T. i., 

 fig. 8. Wm. Sm., B. D., Yol. ii., p. 19, PL xxxiv., fig. 292. Ealfs, in 

 Pritch., p. 773. 



Eiver Dodder, Basin of Grand Canal, Co. Dublin. 



Denticula mutalilis, ("Wm. Sm.) Fresh water. 



Frustules varying greatly in size, generally forming long filaments ; 

 on side view nearly oval, and sometimes narrow, elliptical ; costte 

 broadly marginal, without interstitial striae. (PL 27, fig. 17.) 



Odontidium mutabile, "Wm. Sm., B. D., Yol. ii., p. 17, PL xxxiv., 

 fig. 290. Grunow, Yerhand der K. K. Zool. Bot. GeseL, Band xii., 

 1862, p. 369. — Fragilaria mutabilis, Eab. Fl. Eur., sect. 1, p. 118. 

 Diatoma mutabile, Heiberg, De Danske Diat., p. 58. Cleve, Om 

 Svenska och Norska Diat., p. 219. — Dimeregramma mutabile, Ealfs, in 

 Pritch., p. 790. 



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