O’Mrara—Report on the Irish Diatomacee. 309 
Genus VIII. Navicura, Bory. 
Frustules 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 strie are moniliform, in the latter costate. Considerable 
difference of opinion has existed as to whether or not this distinction 
is tenable. Kiitzing 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 coste 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 1s often very difficult to distinguish 
between stric and coste. 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 striz, in this case, as founded on 
insufficient observation. He says, ‘‘ The so-called costz in the Pinnu- 
larize are quite distinct from the ribs of other genera of Diatomacee, 
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.’’—Ueber 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 
Navicula 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 striz between the coste, 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 ‘‘ Untersuchungen ueber Bau und Entwicklung 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 coste are depressions on the surface 
ot the valve. 2nd. The valves themselves are unsymmetrical. 3rd. 
The arrangement of the cell-contents exhibits a marked difference 
from those of N avicula, as well in the normal condition as also in the 
process of self-division.—Regarding 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. 
