KONGL. SV. VET. AKADKMIENS II ANDLINOAU. HANI) 26. N:() 2. 113 ' 



intro(ln(>ed a classification, founded on the relative nunibei' of the transverse and the longitudinal . 



or obli([ue stri;c. This classification has been adopted by I'kraoallo, who publislied IJSIH (in JJia- 

 tomiste) a monograph of Pleiirosigina and the allied genera Donkinia, Rhoicosigma and Toxoniih'a. 

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

 transverse and longitudinal stria^ is much too artificial a method, altliough 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 striiB may be. In the type the longitudinal stria-, 

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

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



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



adopt the classification of (tRUNOW and Peragallo. I prefer as bases of classification the outline \ 



of the valve and the flexure of the median line. I 



The variation of the forms in Gyrosigma is very great, and the species pass over 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 Naviculic orthosticha". The peculiar G. sju'dahile has a central nodule, which 

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



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

 habitants of fresh, and salt, water. 



Artificial hey. 



^ j Median line central 2. 



1 — — excentric 19. 



„ j Ends protracted into beaks 3. ! 



1 — not — — 6. 1 



„ . Beaks short and stout G. disfortnm W. Sm. 



Ion": — narrow 4. 



{ 



Valve abruptly attenuated into beaks G. macnim W. Sm 



gradually — — 5. 



, j Valve narrow (B. 0,005 to 0,oi mm.) G. prolongafum W. Sm 



M 



1 — broader (B. 0,oir> to 0,024 mm.) G. Fasciola Ehb. 



Valve linear 12. 



— lanceolate, tapering from the middle 7. 



f Longitudinal strice wider than the transverse 8, 



7. I 



— equidistant with — 9. 



\ — — narrower than the — 10. 



^ f Transverse striae about 14 in 0,oi ram G. attenuatum KiJTZ. 



■ I — — — 17 — G. morale W. Sm. 



f Transverse stria about 14 in 0,oi mm G. Terryanum Pee. 



9. ■; — — 17 to 23 — G. acuminatum KtJTZ. 



I — - about 28 — G. glaciale Cl. 



Transverse stria? about 13 in U.oi mm 11. 



— — — 21 — G. Kiitzinqii Grun. 



10 



■ ' - — — 24 — G. Febigeri Grun. 



— — — 27 — G. diaphnnum Cl. 



-.. ( Central area small G. Strigilis W. Sm. 



' \ — — large, oblique G. Baileyi Grun. 



^^. J Valve about If) times longer than broad G. fcnnifisinitou W. Sm. 



t — — 10 — — — 13. 



..., I Transverse and longitudinal striie equidistant 14. 



\ — striae wider than the longitudinal 15. 



... j Central area large, oblique G. plagiostntnuin Grun. 



■ \ — — small — G. balticiim Ehb. 



Sv. Vet. Akad. Handl. Bd 26. N:o 2. 15 



