32. Arcuate valve view showing prominent transverse lines of the septa 

 of the frustules (appearing as costae); raphe along the ventral mar- 

 gin and in the midregion bent inwardly to form a 'V as seen in valve 

 view; frustules epiphytic on filamentous algae and aquatic 

 plants Epithemia 



PL 7, Fig. 19 



32. Frustules without transverse septa (costae) showing in the valve 

 view 33 



33. Axial field expanded in the midregion, forming a clear area in the 

 valve ornamentation which extends to the ventral margin of the 

 curved frustule; cells usually with concave margin against a sub- 

 strate Amphora 



PI. 7, Fig. 15 



33. Axial field central and small, not expanded as above; frustules form- 

 ing linear colonies in gelatinous tubes, or attached singly at the 

 end of a gelatinous stalk (often found floating free); lunate, with 



a slight swelling in the midregion of the ventral margin Cymbella 



PL 7, Fig. 7 



34. Frustules 'S'-shaped or sigmoid; wall ornamented with transverse and 

 longitudinal striae which make a pattern of intersections Gyrosigma 



PL 7, Fig. 14 



34. Frustules not sigmoid 35 



35. Frustules broadly elliptic, slipper-shaped or boat-shaped in valve 

 view, the margins showing prominent, often short costae; surface 

 of valve undulate; in girdle view elongate but with the sides un- 

 dulate; pseudoraphe often indistinct Cymatopleura 



PL 5, Fig. 9 



35. Frustules without such costae; not undulate, not showing marginal 

 undulations in girdle view 36 



36. Raphe along both margins of the valve, located within a keel 37 



36. Raphe not marginal; keel present or absent 38 



37. Valve sharply bent to form a saddle; raphe in a marginal 



keel Campylodiscus 



PL 6, Fig. 1 



37. Valve arched or twisted, sometimes flat, prominent costae extending 

 from the valve margin toward the smooth, pseudoraphe area Surirella 



PL 5, Fig. 14 



38. Frustule in valve view curved and 'bone'-shaped, one pole distinctly 

 larger than the other; transverse rows of puncta in valve view . Actinella 



PL 6, Fig. 13 



38. Frustules some other shape 39 



39. Valve with 'wings', furnished with a sigmoid keel vertical to the face 

 of the valve; boat-shaped in valve view, '8'-shaped or hour glass- 

 shaped in girdle view Amphiprora 



PL 6, Fig. 12 



39. Valves without a sigmoid keel, some other shape than above in 

 girdle view 40 



40. Pseudoraphe on one valve; true raphe on the other 41 



40. Raphe in both valves 43 



[946] 



