KONGL. SV. VET. AKADEMIENS HANDLINGAE. BAND 26. N:0 2. 11 



20. i ^'^'^^ ^'*'^ lateral areas uniting with the central area Nav. lyratm (Anomoeoneis partim). 



I — without — — — — 21. 



81. I P""'^'^ arranged in decussating rows . ... Nav. decussatce (Anomoeoneis partim). 



I — — transverse striae ... . ... .28. 



gg I Striae minutely and transversely lineate . . . gg. 



I — punctatae — — .... ... 24 



( Central nodule elongated . . . . : Brebissonia. 



I not . . . Nav. lineolatce. 



I Puncta arranged in longitudinal, almost straight rows ... ... . . . 25. 



I — — — — undulating rows . 28. 



f Median line between siliceous rihs . . . . 26. 



I not — — — . Nav. orthostichce (Nav. fusiformes partim). 



I Punotation coarse Cistula. 



\ — fine . , .27. 



f Central nodule uniting with the ribs in a very elongated axial costa . . Amphipleura. 

 27. 1 Central nodule not uniting with the ribs or uniting for a short way only . . Frustulia 



I (Nav. orthostichce partim). 

 „„ j Striae distinctly punctate 29. 



23. 

 84. 

 25. 

 26. 



r 



indistinctly or very finely punctate 32. 



29 / ^""°** coarse . . . . Nav. punctatce. 



\ — small .... . . 30. 



gp ( Median striae of unequal length ... . . . Nav. heterostichas. 



' I — — equal — . 31. 



{Valve very convex, and carinated . Tropidoneis. 



Valve slightly convex not carinated . . . Nav. microstigmaticcB (Nav. mesoleice and 



Nav. entoleim partim). 

 ng f Terminal nodules incrassate or transversely dilated .... Nav. bacillares. 



' \ — not — — — 33. 



„„ / Median striae more distant than the others Nav. decipientes (Nav. microstigmaticce partim). 



■ \ — not — — — .... 34. 



„ , f Axial area more or less lanceolate Nav. entoleiae. ' 



\ — indistinct ... ... .... . . 35. 



„. / Central area large . . ... ... . , . . 36. 



{' 



indistinct . . . . . ..... . 37. 



„„ / Central area a transverse fascia . . . . Stenoneis. 



\ — quadrate or rounded . . . Nav. mesoleice (Anomoeoneis partim). 



„„ ( Broad membranaceous forms Nav. minuscules. 



\ Narrow — — .... Nav. fusiformes (Nav. microstigmaticce partim). 



„Q I Valve clavate . .... 39. 



38- ■[ _ „„t - .... . , , . 40. 



„q I Structure: transverse striae or rows of puncta . . Gomphonema. 



I — — costae, alternating with double rows of puncta Gomphoneis. 



, j Structure: decussating rows of minute puncta . . ... Toxonidea. 



\ — transverse striae or rows of puncta . . 41. 



Valve not strongly asymmetrical . . Cymbella (Tropidoneis, Trachyneis, 



41. <J Nav. punctatce partim). 



strongly asymmetrical . 42. 



line on an elevated keel . . Auricula. ^ 



not — — . . . Amphora. 



,„ f Median lir 

 42. ■! 



Habitat and geographical distribution of diatoms. 



A knowledge of the habitat and the geographical distribution of the diatomaceee is of great 

 importance, especially for geological researches. Fossil diatoms occur doubtless more frequently 

 in geological strata than is usually believed, and may furnish the geologist with valuable evidence 

 about the qualities of the water in vehich the sediments were deposited, whether fresh, brackish or 



' Navicula americana. — ^ Tropidoneis partim and Amphora Schmidtii. 



