KONGL. SV. VET. AKADEMIENS ITANDLINGAH. BAND 26. N:0 2. 143 



a sigmoid median liiie occuring in the groups Lmvistriatce {N. Maeana), Caloneis (C. stauroplhora), 

 Lineohifff, Eitroecoiicif: etc. Moreovei' tliere are gradual transitions from forms witli sigmoid 

 median line to forms witli straight median line and terminal fissures in contrary directions [N'. 

 auJdanäica) while many snoh forms are closely connected with others having the terminal fi.ssures 

 in the same direotion. Tn ^V. tnniiihi some speeimens oconr in wliieli tlie median line is scareely 

 sigmoid. 



As to Rhoironei.<i, this genns is also inadmissible, as it eontains widely different forms, and 

 the degree of flexiire in the fnistnle varies in the same species. 8nme forms of the section Lhico- 

 laice are more or less arcnate, and in Gyrosigma and Pleurosigma Ave meet with species, wliich in 

 some varieties are straight, in others arcnate. 



Mif rostig))! a is donhtless akin to Liliellus and lias also a resemblanoe to tlie symmetrieal 

 forms of Tropidoneis, ^vhich differ principally l)y their highly elevated valves. Among the other 

 groups of Navicnla, Microstigma lias affinities with the Decipientes and FusifoiDies, the former 

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

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

 trace any absolute limit between these groups of forms. 



Artificial kei/. 



.. ) Central iiodule stauroid 3. 



■ i — — not 29, 



,, J Stam-os bitid 34. 



^ \ — not , 3. 



., J Enils of the valves with dia])liragms 17. 



■ ) — — — withont 4, 



. j Ends protracted 5. 



' \ — not — 7. 



I Stauros broad S. dilatata Ehe.. 



5. -"i ,. 



^ — narrow 0. 



( Stria; 15 in 0,0 1 iiim S. Pliyllndes Eiib. (S. iniccps var. iinbilis). 



' \ — 30 to 30 in 0,01 mm S. anccps Ehb. 



f Size small (L. 0,oi3 to 0,035 mm.) 8. 



7. ■! — median (L. about 0,0,'i mm.) 10. 



I — large (L. 0,0 7 to 0,2 mm.) IG. 



[ Linear-elliptical .9. septentrionalis Gkun. 



8. I Laneeolate S. perpusilla Grun. 



I Elliptical 9. 



(, J Stri» about 18 in (J,oi mm S. kryophila Grun. 



' ' \ — — 32 — (S. perniiniita Grun. 



( Ends broad, capitate or truneate 11- 



I — obtuse or rounded 12. 



in .' 



' — cuneate 8. Demerarce Cl. 



— subacute 15. 



I Strise 14 to 19 in 0,oi mm S. desiderata Cl. 



' \ — 39 — — S. pacliycephala Cl. 



.j, I Stauros small and short S. piellucida Cl. 



I™-. \ . ^o 



[ — pervious io- 



.-, J Zone broad with numerous distinct divisions S. Biblos Cl. 



I — narrow with faint divisions 14. 



.. I Stauros broad S. Gregorii Ralps. 



\ — narrow S. constricta W. Sm. 



.jr j Striffi 17 to 18 in 0,oi mm 8. salina W. Sm. 



^"'- I _ 33 _ _ S. africana Cl. 



I Linear S. Schinzii Brun. 



'■ 1 Rhomboid-lanceolate S. Phoenicenteron Ehb. 



..„ j Apiculate 8. Smithii Grun. 



I Non-apiculate 18. 



