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



and most exact hitherto published. For that reason I have in 

 most cases of uncertainly about the signification of the older na- 

 mes followed the views accepted in the last named classical work. 



Although this work gives a valuable help in determining the 

 forms of Pinnularia, there still remain great difficulties, which may 

 be got rid of only by a careful study of the minute characteri- 

 stics, such as the structure of the raphe, the number of strise, the 

 peculiarities of the end-fissures, the breadth of the band across 

 the striae, which bands are occasioned by the cavities on the in- 

 side of the valve 1 ). These characteristics, together with the form 

 and size of the valve, as well as the breadth of the area, or 

 unstriated part of the valve, have guided me in limiting the species. 



As to the raphe it seems to be formed by a more or less 

 oblique, and sometimes twisted, fissure. Among the larger species 

 of Pinnularia two types may be distinguished. In the one the 

 raphe forms simply an oblique fissure, in the other the raphe has 

 a more complicated structure and seems to be twisted *). The 

 former type of raphe is most developed in P. transversa (A. S. 

 Atlas XLIII. 5, 6), the latter in the species named below P. strep- 

 toraphe (A. S. XLII, 7). I have always found the raphe oblique 

 in P. major and in the true P. Dactylus, complex in P. nobilis, 

 P. gentilis, P. viridis etc. In some American deposits (Cherryfield, 

 Crane Pond) there occur forms, similar to P. Dactylus, but with 

 complex raphe. I regard them as belonging to another species 

 (P. flexuosa Gl. mpt). 



The end-fissures are generally similar in both ends of the 

 valve, but in P. Flamma A. S. and P. Flammula A. S. from De- 

 merara they are turned in opposite directions. The same is the 

 case with the interesting P. platycephala Ehb., which belongs to 

 the flora of Finland. 



The number of striae is more constant than is generally be- 

 lieved and is a valuable characteristic. 



P. platycephala Ehb. — M. G. XVII, II, 9. — Valve linear, 

 5 to 6 times longer than broad, with broad, subcapitate ends and 

 slightly undulate margins. Raphe forms a straight, narrow line 



x ) See Otto Muller: Durchbreclmngen tier Zelhvand in ihrem Bez. zu 

 Ortsbewegung der Bacillarien. Ber. d. Deutsch. Bot. Ges. VII, 4, 1SS9. 



