ANALYTICAL KEY TO THE GENERA 



231 



34- 

 34- 



35- 

 35- 



36. 

 36. 



37- 

 37- 

 38. 

 38. 



39- 

 39- 

 40. 

 40. 



4i- 

 41. 



42. 



42. 



43- 

 43- 

 44. 



44. 



45- 

 45- 

 46. 

 46. 



47- 

 47- 

 48. 

 48. 

 49. 

 49. 



5°- 



5°- 



5 1 - 

 5 1 - 



52- 



52- 



53- 

 53- 

 54- 

 54- 



Each angle of the cell bearing one long bristle 

 Angles of the cell bearing processes 



Processes cornutate 



Processes simple boss-like projections ... 



Processes cornutate terminated with short spines 

 Processes cornutate without spines 



Cells in curved, ribbon-like chains ; valve bearing a small eccentric spine 

 Cells in chains with a slight spiral twist 



No intercellular spaces; cells weakly siliceous.. 

 Intercellular spaces large and elliptical... 



Cells solitary 



Cells united in chains 



Valves with a long central spine 



Valves without central process of any kind 



Cells with projections or differentiated areas in the angles of the valves 



... Chaetoceros (289) 



35 



... 36 



37 



Hemiaulus (285) 



... Biddulphia (273) 



Eucampia (285) 



38 



... Streptolheca (287) 

 ... Climacodium (287) 



40 



43 



Ditylum (284) 



4 1 



42 



Valve surface flat ; no projection or differentiated area in the angles of the valves ; several 

 indistinct radial lines terminating in marginal apiculi ... ... Pseudo-triceratium (282) 



Valve surfaces hexagonally areolate ; areolation transverse ; processes cornutate 



Triceratium (283) 

 Areolation radial; process a boss of fine pores... ... ... ... ... Trigonium (280) 



Connective zone continuous from cell to cell; valve surfaces contiguous ... Bellerochea (280) 

 Valve surfaces not contiguous ... ... ... ... ... ... ... Lithodesmium (*) 



Cells with more or less bilateral symmetry upon the apical axis in the valvar plane, median 

 area bearing a raphe, i.e. a true cleft upon each valve 

 Cells with raphe not in median area upon each valve... 



Cells in valve view linear to oval-lanceolate, axile area straight 

 Valves sigmoid, sublunate or spirally twisted ... 



Cells with laminate valves 

 Valves not laminate 



Valve punctate, striation reaching margin of valve 



Striation not reaching valve margin, leaving hyaline lateral spaces 



Cells with sigmoid flexure 



Cells sublunate or spirally twisted 



Cells sublunate, raphe close to ventral margin of the cell 

 Cells with torsion about apical axis 



Cells with alate projections; zone complex; raphe sigmoid, not in the median line 



Amphiprora (349) 

 Raphe in the median line, straight in valve view, zone simple ... ... Tropidoneis (350) 



45 



5 1 



46 



48 



Trachyneis (346) 



47 



Navicula (343) 

 Scoresbya (346) 



Pleurosigma (347) 



49 



Amphora (351) 

 5° 



Raphe carried upon an eccentric keel ... 

 Cells without a true raphe upon each valve 



Cells with raphe on one valve and pseudoraphe on the other 



Cells without a true raphe upon either valve ... 



Valves symmetrical about apical and transapical axes... 



Valves asymmetrical about apical axis ... 



Cells with median pseudoraphe upon both valves 



Cells without raphe or pseudoraphe 



* See note on page 229. 



Nitzschia (351) 

 52 



53 



54 



Cocconeis (341) 

 Achnanthes (340) 



55 



63 



