KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 



26. 



N:o 



113 



introduced a classification, founded on the relative number of the transverse and the longitudinal 

 or oblique strife. This classification has been adopted by Peragallo, who published 1891 (in Dia- 

 tomiste) a monograph of Pleurosigma and the allied genera Donkinia, Rhoicosigraa and Toxonidea. 

 — It seems to me that to separate the species in accordance with the relative number of the 

 transverse and longitudinal striae is much too artificial a method, although this characteristic may 

 in many cases be useful. Gyros. Fasciola offers a striking illustration of how unnatural such a 

 classification based on the relative number of the striae may be. In the type the longitudinal striae 

 are closer than the transverse, but in the var. sulcata the transverse are closer than the longi- 

 tudinal striae. In some cases moreover the relation between the transverse and longitudinal striae 

 may be vitiated by inevitable errors in their counting. These reasons have induced me not to 

 adopt the classification of Grunow and Peragallo. I prefer as bases of classification the outline 

 of the valve and the flexure of the median line. 



The variation of the forms in Gryrosigma is very great, and the species pass over into each 

 other in so many cases that it is very difficult to define them. 



Gryrosigma is related to Tropidoneis by the carinated forms (Donkinia). On the other hand 

 it is related to the Naviculae orthostichae. The peculiar G. spectabile has a central nodule, which 

 closely resembles that of the above named section (as of Nav. cuspidata). 



The majority of forms, belonging to Gryrosigma live in brackish water, but a few are in- 

 habitants of fresh, and salt, water. 



Artificial hey. 



^ { Median line central 

 ' \ — — excentric 

 _ ( Ends protracted into beaks 



\ — not — — 

 „ f Beaks short and stout . . 

 3- I _ 



{■ 



long — narrow 



Valve abruptly attenuated into beaks . . 



gradually — — 



j Valve narrow (B. 0,oo5 to 0,oi mm.) . . . 

 \ — broader (B. 0,oi5 to 0,024 mm.) . . 



{ Valve linear 



\ — lanceolate, tapering from the middle 



{Longitudinal striae wider than the transverse 

 — — equidistant with — 



— — narrower than the — 



„ f Transverse strise about 14 in 0,01 mm. . 

 ^- i - _ _ 17 - 



f Transverse strise about 14,in 0,oi ram. 



_ 17 to 23 — 



[ — — about 28 — 



{Transverse striae about 13 in 0,0 1 mm. 

 _ _ — 31 — . . 



- _ - 24 - 



_ - — 27 — 



( Central area small ■ 



\ — — large, oblique 

 j Valve about 16 times longer than broad 

 ^- \ - - 10 _ _ - 



... f Transverse and longitudinal striae equidistant 

 ' \ — striae wider than the longitudinal . 



i Central area large, oblique 



' \ — — small — 



K. Sv, Vet. Akad. Handl. Bd 26. N:o 2. 



11. 

 12. 

 13. 



2. 



19. 



3. 



6. 



G. distortum W. Sm. 



4. 



G. macrum W. Sm. 



5. 



G. prolongatum "W. Sm. 



. . . G. Fasciola Ehb. 



... 12. 



... 7. 



... 8. 



9. 



... . 10. 

 G. attenuatum Kutz. 

 . . G. litorale W. Sm. 

 . G. Terryanum Pbb. 

 . G. acuminatum KtJiz. 

 G. glaciale Cl. 

 . . 11. 

 G. Kiitzingii Gkun. 

 . . G. Fehigeri Gkun. 

 . Gr. diaphanum Cl. 

 G. Strigilis W. Sm. 

 . . G. Baileyi Gedn. 

 tenuissimum W. Sm. 

 13. 

 .... 14. 



... . . 15. 



G. plagiostomum Gkun. 

 G. balticnm Ehb. 



G 



15* 



