188 



1. 



2. 

 3. 



4. ( 



I Low 



'■{ 



10. 

 11. 

 13. 



Artificial key. 



Valve biconstricted 



not constricted 



Lower valve with a transverse fascia . . 



— without .... 



Valve narrow, linear-lanceolate . . 



— narrow, elliptical .... 



— elliptical, rostrate . . 



Valve broad and short 



— narrow and elongated 



Striffi fine (25 in 0,oi mm.) .... 



— coarse (10 to 14 in O.ol mm.) 



Valve broadly lanceolate ... . . 



— elliptical 



Striae 12 to 16 in 0,oi mm 



— 21 or more 



Valve lanceolate 



linear 



StrisB 21 in 0,01 mm 



— 24 or more in 0,01 mm. . . . . . 



Valve with parallel margins 



— — slightly convex . . 

 Fresh water habitat ... 

 Marine .... ... 



Ends subcapitate .... 



— obtuse 



I Valv 



. . . A. trinodis Arn. 



2. 



3. 



4. 



.A. af finis Gbon. 

 . . A. hungarica Geun. 

 . . . A. exigua Gkun. 



5. 



7. 



A. Biasolettiana Kt'TZ. 



6. 



. . A. delicatula Ktiiz. 

 . A. Hauckiana Grun. 



9. 



. . A. Hudsonis Geun. 



. ■ . A. gldbrata Grun. 



.A. eocilis Kutz. 



10. 



11. 



12. 



. A. linearis W. Sm. 



. A. twniata Geun. 



A. microcephala KijTz. 



. A. minutissima Ktiiz. 



1. A. ininiitissima Kutz. (1833). — Linear with somewhat attenuate and obtuse ends. 

 L. 0,015 to 0,02; B. 0,003 to 0,004 mm. Upper V. with narrow, linear axial area. Striae about 

 26 in 0,01 mm. parallel. Lower V. without axial area and with small, transverse central area. 

 Strise about 25 in 0,oi mm., stronger in the middle, slightly radiate. — Dec. N:o 75 (1833) accor- 

 ding to Lagst. Grun. A. D. p. 23. V. H. Syn. p. 131 PL XXVII f. 35 to 38. A. exilis W. Sm. 

 B. D, II PI. XXXVII f. 303. 



Fresh water: Sweden! Germany! England! Belgium (V. H.), probably common everywhere. 



Var. cryptocephala Gron. — Smaller, with slightly capitate ends. L. 0,oi 2 to 0,oi8; B. 0,oo2 

 to 0,003 mm. Strise about 30 in 0,oi mm. — V. H. Syn. PI. XXVII f. 41 to 44. 



Fresh water: Finland! 



2. A. microcephala KtJtz. (1844). — Linear lanceolate, with subcapitate ends. L. 0,oo9 

 to 0,026; B. 0,003 mm. Upper valve with linear axial area and parallel strise, 30 — 36 in 0,oi mm., 

 the median strise abbreviated. Lower V. without axial area and with small transverse central 

 area. Strise about 32 in 0,oi mm. slightly radiate. — A. microceplialum KtJTZ. Bac. 75 PI. Ill 

 f. 13, 19 (1844). Grun. A. D. p. 22. V. H. Syn. p. 131 PI. XXVII f. 20 to 23. Navicala 

 pachycepJiala Schum. Preuss. D. 2:te N. II f. 44? 



Fresh water: Sweden (Helsingland) ! Belgium (V. H.). 



Achnanthidium microcephalum W. Sm. B. D. is probably Anomoeoneis exilis Grun. 



3. A. linearis W. Sm. (1855). — Linear with rounded ends. L. 0,oi to 0,02; B. 0,oo3 to 

 0,004 mm. Upper V. with narrow, linear axial area and parallel strise, 24 to 27 in 0,oi mm. 

 Lower V. without axial area and with small and transverse central area. Strise 22 to 28 in 0,oi 

 mm. — Achnanthidium lin. W. Sm. Ann. and Mag. Nat. Hist. 1855 p. 8 PI. I f. 9. B. D. II 

 p. 31 PL LXI f. 381. Achnanthes lin. Grun. A. D. p. 23. V. H. Syn. p. 131 PL XXVII f. 31, 32. 



Fresh water: Norway (Dovre)! England! Belgium (V. H.). 



