KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 26. N:0 2. 113 



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

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

 tomiste) a monograph of Pleurosigma and tbe allied genera Donkinia, Rhoieosigma and Toxonidea. 

 — It seems to me tliat to separate tbe species in accordance with tlie relative number of the 

 transverse and longitudinal striEe is mucli too artificial a metliod, although this charaeteristic may 

 in many cases be usefnl. Gyros. Fasciola oifers a striking illiistration of how unnatural sucli a 

 classification based ou the relative n^Tmber of the striee may be. In the type the longitudinal striae 

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

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

 may be vitiated by inevitable errors in their counting. These reasons have iudueed me iiot 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 Gyrosigma is very great, and the species pass över into each 

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



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

 it is related to the JSTaviculai orthostichse. The peculiar G. spectahile has a central nodule, which 

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



The majoritj' of forms, belonging to Gryrosigma live in brackish vpater, but a few are in- 

 habitants of fresh, and salt, water. 



Artificial key. 



.. ( Median line central 2. 



' \ — — excentric 19. 



„ I Ends protracted into beaks 3. 



\ — not — — 6. 



„ I Beaks short and stont G. distortum W. Sm. 



\ — long — narrow 4. 



. ) Valve abruptly attenuated into beaks G. macriim W. Sm. 



1 — gradually — — 5. 



r J Valve narrow (B. 0,oo5 to 0,oi mm.) G. i^rolongatiim W. Sm. 



{ 



M 



— broader (B. 0,oiö to 0,024 mm.) G. Fasciola Ehe. 



Valve linear 12. 



— lanceolate, tapering from the middle 7. 



i Longitudinal strise wider than the transverse 



7. < — — equidistant with — 9. 



\ — — narrower than the — 10. 



^ j Transverse striee abont 14 in 0,0 1 mm Q. attenuatum Kfiiz. 



■ I — — —17 — G. litorale W. Sm. 



("Transverse striae abont 14_in 0,01 ram G. Terryanum Per. 



9. •! — — 17 to 23 — G. acuminatiim Kutz. 



I — — abont 28 — G. glaciale Cl. 



I Transverse stria abont 13 in 0,0 1 mm 11. 

 — — — 21 — G. Kiitzmgii Grun. 



^'■'- ' — — — 24 — G. Fehigeri Grun. 



27 — G. diaphanum Cl. 



.... 1 Central area small G. Strigilis W. Sm. 



( Central area 



large, oblique G. Baileyi Grdn, 



.,„ J Valve abont 15 times longer than broad G. femiissimitm "W. Sm. 



^•^^ \ - - 10 - - - 13. 



w., J Transverse and longitudinal strise equidistant 14. 



1 — striae wider than the longitudinal 15. 



.. , J Central area large, oblique G. plagiostomtmi Grun. 



' ) — — small — G. baltinim Ehb. 



K. Sv. Vet. Alfcid. Handl. Bd 26. N:o 2. 15 



