KONfiL. SV. VET. AKADEMIKNS 11 AKDLINCiAH. I5AN1) 26. N:() 2. 143 



a sigmoid median line occuriiig in tlie groups Lrevisfridfa- (N. Racanrt), (Jaloncis {(]. staicrnphora). 

 Lincoldfrr, Eurocconris etc. Moreover there arc gradual transitions from forms witli sigmoid 

 median line to forms with straight inedian line and terminal tissnres in contrary directions (iV. 

 nulilandica) while many such forms are closely connected with others having tlie terminal fissures 

 in the same direction. Tn N. fitniida some specimens occur in which the median line is scarcely 

 sigmoid. 



As to Blioiconeis, this genus is also inadmissihle, as it contains widely diffei'ent forms, and 

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

 l((fcc are more or less arcuate, and in (lyrosigma and Pleurosigma we meet with species, wliicji in 

 some varieties are straight, in others arcuate. 



Microstigma is doulitless 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 Dccipicntet^ and Fusifortncs, the former 

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

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

 trace any absolute limit ])etween these groups of forms. 



Artificial hey. 



^ I Central nodule stauroiil ^ 



{ 



10. 



11. 

 12. 

 13. 

 14. 

 If). 

 16. 



— not 29. 



Stauros bifid 24. 



— not 3. 



j Ends of the valves with diaphragms 17. 



\ — — — without 4. 



J Ends protracted 5. 



\ — not — 7. 



J Stauros broad S. dilatata Ehb.. 



\ — narrow (J. 



J Striae 15 in 0,(n mm S. Pltyllodes Ehb. [S. anceps var. nobilis). 



\ — 20 to 30 in 0,01 mm S. anceps Ehb. 



f Size small {L. 0,oi3 to 0,02r. mm.) 8. 



< — median (L. about 0,o.'> mm.) 10. 



I — large (L. 0,07 to 0.2 mm.) IG. 



Linear-elliptical S. septentinonalis Grun. 



Lanceolate S. perpusilla Grun. 



Elliptical 9. 



Strife about IH in O.oi nun S. kryophila Grun. 



22 — S. perminuta Grun. 



Ends broad, capitate or truncate 11. 



— obtuse or rounded 12. 



— cuneate S. Deinerarce Va,. 



— subacute 15. 



j Striae 14 to 19 in O.iii mm S. desiderata Ol. 



\ — 29 — — S. j)arhycephala Cl. 



Stauros small and short S. pellucida Ol. 



pervious 13. 



j Zone broad witli numerous distinct divisions <S'. Bihlos Cl. 



narrow with faint divisions 14. 



Stauros broad ,S'. Gregorii Ralfs. 



narrow S. constridn W. Sm. 



j StriiB 17 to 18 in 0,oi mm S. snlina W. Sm. 



1—23 — — , S. africana Cl. 



I Linear S. Schinzii Brun. 



I Rhomboid-lanceolate S. Phoenicenferon Ehb. 



j Apiculatp S. Smithii Grun. 



1 Nnn-ajjicuiate 18. 



( Stria 



