386 KEV EUGENE O^MEARA. 



In this genus two mother-cells co-operate to produce two 

 auxosporesj as appears from the observations of De Bary, who 

 saw N.firmum in '^copulation." In this new genus are placed 

 Navicula firma (Kiitz,), N. Amphigomphus (Ehren.), N. affinis 

 (Ehren.), N. limosa (Klitz.), forms included in Grunow's 

 natural group of Limosa. Whether the circumstances re- 

 ferred to are sufficient to justify the establishment of a new 

 genus, I leave my readers to form their own opinion. 



Pinnularia (Ehren.). — This genus was originally distin- 

 guished from Navicula by the fact that the striae in the 

 former are uninterrupted or costate, as is the common 

 designation, whereas in the latter they are resolvable into 

 dots. This distinction some later writers consider insuffi- 

 cient, as Grunow, Schuman, Ralfs, Heiberg, Cleve, who 

 have therefore included the sjjecies of Pinnularia in the 

 genus Navicula. Pfitzer, however, rfe-establishes Pinnularia 

 as an independent genus. His observations on the subject 

 are worthy of consideration. The so-called costate striae of 

 the Pimmlarice were thought by Dippel to be thickened and 

 elevated portions of the siliceous covering. Pfitzer, on the 

 contrary, regards them as depressions of the outer surface of 

 the valve. The structure, though quite observable in the 

 larger forms, as he thinks, such as P. lata, is not so manifest 

 in the smaller species, and for this reason he regards the 

 distinction as doubtful. Another distinction attributable to 

 Schuman is noticed, namely, that in the Navicula the stria? 

 in one direction are of a uniform character, whereas in the 

 Pinnularia, between the deep broad strise, fine lines are 

 interposed. These finer lines referred to by Schuman Dr. 

 Pfitzer has never been able to discover, and my experience 

 coincides with his. 



The characters on the strength of which the independence 

 of the genus Pinnularia is maintained are the unsymmetrical 

 nature of the valves and the peculiar construction of the 

 endochrome plates in the act of division. The former of 

 these characters is open to doubt, as the valves appear gene- 

 rally as symmetrical as those of the Navicula ; the latter 

 feature, however, is noteworthy. In Pinnularia the en- 

 dochrome plates move from the girdle-bands across the 

 valve, as is the case in the Navicula; but, as occurs in 

 Neidium, the fission takes place from the ends in a direction 

 parallel to the longitudinal axis. In this genus two mother- 

 cells produce two auxosj)ores. 



Stauroneis (Ehren.). — This genus is distinguished from 

 Navicula by the transversely expanded middle nodule, but 

 corresponds with it in the disposition of the cell contents. 



