14 Acta Societatis pro Fauna et Flora fennica. VIII, n. 2. 



Amphiprora and Pleurosigma is formed by Donkinia. Pleurosigma 

 is closely connected with some species of Navicula (N. O'Mearii 

 Grun, N. Kjellmanii CI., N. vitrea CI.). Between the last named 

 forms and the other Navicular there is a gap. Only a few forms 

 of the Navicula directa-group (N. Kariana) remind us of them. 

 Perhaps is Amphiprora akin also with the group of Naviculse 

 complexae (N. Libellus). 



The large genus Navicula comprises such a number of wi- 

 dely different forms, as to afford sufficient reasons for splitting it 

 up into different genera. On the other hand many forms now 

 placed in the genera Cymbella, Stauroneis, Rhoiconeis etc., are 

 more nearly related to the Navicular than many species of the 

 latter are to each other. 



Many characteristics, on which genera and groups are foun- 

 ded, are only of specific value, sometimes of such slight signifi- 

 cance that only varieties may be founded on them. For instance 

 the dilated central nodule, which is regarded as a characteristic 

 feature of Stauroneis, the often slightly asymmetric form of the 

 valve, on which Cymbella and Alloioneis are founded, the direction 

 of the end-fissures of the raphe, the sigmoid, bent, or arcuate 

 form of the valves, on which Scoliopleura and Rhoiconeis are 

 founded, and many other such distinctions. 



Pfitzer 1 ) separates from Navicula as distinct genera: Pin- 

 nularia, Nehlium, Anomoeoneis, Friistulia, Colletonema and Schi- 

 zonema. 



The first named genus, Pinnularia, has so many peculia- 

 rities, that no doubt can exist about the correctness of separa- 

 ting it from Navicula. 



Neidtum comprises the group of Nav. affinis and of Nav. 

 limosa. If the latter be excluded Neidium is a very natural and 

 distinct group of closely allied forms. 



Anomoeoneis comprises the groups of Nav. sculpta and Nav. 

 serians, and may also be regarded as a natural genus. 



Friistulia is the same genus as Van Heurckia, but an ol- 

 der name. Colletonema resembles Friistulia in all respects except- 

 ing that the frustules are enclosed in gelatinous tubes or mas- 



'; Dnters. iiber Ban und Entwicklung der Bacillarieen. Bonn 1871. 



