O'Meara — Rejjort on the Irish Diatoinacece. 339 



Genus YIII. ISTayicula, Boiy. 

 Trustnles simple, free. 



Ehrenberg separated the forms included in this genus into two dis- 

 tinct genera, Navicula and Pinnularia, founded on the fact that in the 

 former the striae are moniliform, in the latter costate. Considerable 

 difference of opinion has existed as to whether or not this distinction 

 is tenable. Kijtzing rejected it, while Wm. Smith and Rabenhorst 

 maintained its validity. Ralfs, in Pritch., p. 892, included the species of 

 Pinnularia under the genus Navicula for the following reasons : — 

 '' Were the costse always plainly developed, as in Pinnularia nobilis and 

 its allies, no difficulty could occur in determining the genera ; but in 

 many of the more minute species it is often very difficult to distinguish 

 between strife and costa3. We have not admitted Pinnularia here, 

 partly for the reason just given, but principally because we cannot 

 decide to which genus a large number of Ehrenberg's species should 

 be referred." The existence of the distinctive characteristic is here 

 admitted, but the genus founded upon it is discarded on account of the 

 difficulty of applying it in many cases. Grunow regards the distinction 

 between costate and moniliform strise, in this case, as founded on 

 insufficient observation. He says, " The so-called costse in the Pinnu- 

 lariae are quite distinct from the ribs of other genera of Diatomaceae, 

 and consist of a union of more or less confluent puncta, which cannot, 

 indeed, be clearly discriminated, except by the help of good amplifica- 

 tion and well-managed illumination." — TJeber neue oder ungeniigend 

 gekannte Algen, Verhand der K. K. Zool. Bot. Gesel., Band x., 1860, 

 p. 513. This eminent author thus discards the distinction between 

 Ifavicula and Pinnularia, and is followed by Heiberg, Cleve, and 

 others. Schumann, who adopts the same view, indicates a peculia- 

 rity in some of the larger forms of Pinnularia, (P. nobilis and P. major, 

 for example,) which is worthy of special notice here, namely, the 

 interposition of very fine striae between the costse, which he says are 

 indistinct in P. nobilis, but quite distinct in P. major ; these intersti- 

 tial markings I have never been able to discover, and Pfitzer makes 

 the same remark concerning them. The last-named author, in his 

 treatise " ITntersuchungen ueber Ban und Entwickking der Bacillaria- 

 ceen" maintains the distinctiveness of the genus Pinnularia, not on the 

 ground of the different character of the striation, but on the following 

 peculiarities : — 1st. The so-called costse are depressions on the surface 

 of the valve. 2nd. The valves themselves are unsymmetrical. 3rd. 

 The arrangement of the cell-contents exhibits a marked difference 

 from those of Navicula, as well in the normal condition as also in the 

 process of self-division. — Pcgarding the characteristics just named, 

 some remarks are here required. As to the first, supposing it to be 

 true, there is great difficulty in applying it in the more minute forms. 



