K0N6L. SV. VET. AKADEMIENS HANDLINGAK. BAND 26. N:0 2. 143 



a sigmoid median line occuring in the groups Lcevistriatee (N. Raeana), Caloneis {G. staurophora). 

 Lineolatee, Eucoceoneis etc. Moreover there are gradual transitions from forms with sigmoid 

 median line to forms with straight median line and terminal fissures in contrary directions {N. 

 auhlandica) while many such forms are closely connected with others having the terminal fissures 

 in the same direction. In -A^. tumida some specimens occur in which the median line is scarcely 

 sigmoid. 



As to Rhoiconeis, this genus is also inadmissible, as it contains widely different forms, and 

 the degree of flexure in the frustule varies in the same species. Some forms of the section Lineo- 

 latrc are more or less arcuate, and in Gyrosigma and Pleurosigma we meet with species, which in 

 some varieties are straight, in others arcuate. 



Microstigma is doubtless akin to Libellus and has also a resemblance to the symmetrical 

 forms of Tropidoneis, which differ principally by their highly elevated valves. Among the other 

 groups of Navicula, Microstigma has affinities with the Decipientes and Fusiformes, the former 

 having the median striae wider (as in N. tumida), the latter having the puncta arranged in longi- 

 tudinal rows; and with the Entoleice, which have a distinct axial area. In fact it is impossible to 

 trace any absolute limit between these groups of forms. 



Artificial hey. 



^ ) Central nodule stauroid . . 2. 



1 — — not . . .29. 



g f Stauros bifid 24. 



\ — not 3. 



„ J Ends of the valves with diaphragms 17. 



' \ — — — without 4. 



Ends protracted 5. 



— not — .... . . 7. 



Stauros broad ... 8. dilatata Ehb.. 



— narrow . . 6. 





10. 



11. 

 13. 



f Striae 15 in 0,oi mm,. 8. Phyllodcs Ehb. (S. anceps var. nohilis). 



\ — 20 to 30 in 0,oi mm. . . . 8. anceps Ehb. 



Size small (L. 0,oi3 to 0,025 mm.) . 8. 



— median (L. about 0,05 mm.) ... . . . 10. 



— large (L. 0,07 to 0,2 mm.) 16. 



{Linear-elliptical . . . . <S. septentrionalis Geun. 

 Lanceolate . . . . 8. perpusilla Geun. 

 Elliptical . . 9. 



f Striae about 18 in 0,oi mm. . ... . . . 8. kryophila Geun. 



\ — — 22 — ... 8. perminuta Grun. 



I Ends broad, capitate or truncate 11. 

 — obtuse or rounded 12. 

 — cuneate . ' 8. Demerarce Cl. 

 — subacute . . . . ■ ■ 15. 



f Striae 14 to 19 in 0,oi mm. . . . . . . . . . 8. desiderata Cl. 



\ — 29 — — . • . ■ . . 8. pachycephala Cl. 



f Stauros small and short . . . . . . . 8. pellucida Cl. 



\ — pervious 13. 



„ I Zone broad with numerous distinct divisions 8. Biblos Cl. 



narrow with faint divisions 14. 



^ . J Stauros broad . • -8. Gregorii Ralps. 



\ — narrow ... 8. constricta W. Sm. 



(Zom 



Stau 



( Striae 17 to 18 in 0,oi mm 8. salina "W. Sm. 



•'^- ) _ 23 _ _ ........ . . . . . . . S. africana Cl. 



J Linear ■ 8. 8chinzii Bsun. 



■ I Rhomboid-lanceolate ' .... 8. Phoenicenteron Ewb. 



17. 



j Apiculate ... . . . '.' '■:''''.' 8. 8'mithii Geun. 



\ Non-apiculate . .. i. ........ . . . 18. 



