KONGL. SV. VKT. AKADEMIEN'fS IIANDLINCiAK. BAND. 26. N:0 2. 157 



symmetrical naviculoid forms. On the other hand several speeies in other groups of naviculoiil dia- 

 toms are more or less decidedly asymmetrical. For instance, all the larger forms of Pinnularia shew 

 a tendency to asymmetry. The same is the case with numerous varieties of Tracliyncis aspera. 

 All the Amphome are asymmetrical, and usually in a more decided manner than the ('ymbelkc. 

 From some groups of the old genus Amphora the Cymbellii; differ only by their connecting zone 

 not being complex. From other groups of the same genus the distinction is attended with more 

 difficulty, especially in the new group CymhumpJiora, the valves of which are still more asymme- 

 trical, having the median line close to the ventral margin. But the Cynibamjjhoriu are marine 

 or Ijrackish, while the Cymbellir usually are of fresh-water habitat. I 



The structure of the Cymbella' consists of puncta disposed in rows or of transverse strict', 

 which are more or less transversely lineate. The strife in the middle of the valve are radiate and 

 somewhat more distant than in other parts of the valve. In the ends they are parallel or radiate. 

 They either reach the median line, or end at some distance from it, leaving a narrow axial area, 

 frequently dilated around the central nodule. In some species there is a shallow depression on the 

 ventral side of the central nodule, the median ventral striae appearing to be crossed, below the 

 central nodule, by a semicircular furrow. In one division of Cymbella there is in the middle of 

 the central nodule or on its ventral side a peculiar punctum or pore, for which I use the name 

 stiyma. If the stigma is below the nodule, a fine, fissure-like line proceeds from it into the nodule. 

 The median line is in some species oblique and therefore broad, shewing a complex structure, as 

 in some Pinnularias. The terminal nodules are usually close to the end of the valve, and the 

 fissures, especially in the boat-like forms, reflexed to the dorsal side. In the naviculoid forms it is 

 comma-like, and in C. heJcefIra straight, in the direction of the median line, thus forming a passage 

 to Encyonema, which has the terminal fissures bent downwards. 



The Cymbella} are certainly nearly akin to the NaviculcE of the section Lineolatce. Navicula 

 diccpliala is closely connected with 6'. amphicephala, and C. naviculiformis, and with these a number 

 of forms are more or less related: 



Navicula dicephala 



Cyml)ella amphicephala Cymbella naviculiformis 



C. Ehrenbergii C. tumida C. heteropleura 



^1 I. I . 



C. aspera C. Cistula C. oregonica 



C. lanceolata C. Janischii. 



Gynih. microce2)hala shews some signs of affinity to Navicula Bulnheimii, and we may thus 

 trace passages to a number of forms 



Navicula Bulnheimii 



9 



I. 



Cymbella microcephala 



I 



C. Cesatii 



C. angustata C. stauroneiformis 



I 

 C. austriaca. 



Cymbella alpina is an isolated form, without, so far I can see, any close relation to the 

 Lineolatce, although the structure of the stricB is the same as in the typical form of that group of 

 Navicula. Cymb. sinuata is also an isolated form, which seems to have its nearest relatives in 

 Gomphonema. 



