4. Frustules circular in valve view 5 



4. Frustules oval, broadly elliptic, slipper-shaped or rhomboid 8 



5. Valve with a single, undecorated, m.immillate protrusion or thicken- 

 ing just within the margin; (frustules sometimes broadly elliptical or 

 rhomboidal) Actinocyclus 



PI. 5, Fig. 4 



5. Valve without intramarginal protusion 6 



6. Valve with an intramarginal zone of costae, smooth or finely punc- 

 tate within the costal zone; with various markings in the center 

 zone, or smooth Cyclotella 



PI. 5, Fig. 5 



6. Valve marked by rows of puncta radiating from the center to the 

 margins; frustules drum-shaped, rectangular in girdle view 7 



7. Valves evenly ornamented, the rows of puncta usually forking, sometimes 

 an intramarginal circle of fine teeth; plants euplanktonic Coscinodiscus 



PI. 5, Fig. 7 



7. Valves unevenly ornamented, the radiating rows of puncta separated 

 by clear, smooth, radiating zones; intramarginal circle of coarse 

 spines which extend beyond the edge of the valve; plants euplank- 

 tonic or tychoplanktonic Stephanodiscus 



PI. 5, Fig. 8 



8. Frustules broadly elliptic or oval in valve view, variously shaped in 

 girdle view but commonly short rectangular, the corners protruding 

 and out-turned; no raphe or pseudoraphe Biddulphia 



PI. 5, Fig. 2 



8. Frustules with a raphe or pseudoraphe, oval or rhomboidal in valve 

 view, corners not protruding 9 



9. Frustules rhomboidal to circular in valve view, arched and saddle- 

 shaped in girdle view; pseudoraphe in one valve at right angles to 

 that of the other Campijlodiscus 



PL 6, Fig. 1 

 9. Frustules oval or broadly elliptic, not bent nor saddle-shaped 10 



10. Frustules broadly elliptic or sHpper-shaped, with prominent marginal 



costae; raphe lateral in a marginal keel 11 



10. Frustules oval or narrowly elliptic, with a pseudoraphe, or with a 

 raphe not in a marginal keel 12 



11. Frustules slipper-shaped, surface of valve transversely undulate, seen 

 when the cell is viewed from the side; transverse striae often faint, 

 occurring in zones or bands across the valve; some species much 

 longer than wide Cymatopleura 



PI. 5, Fig. 9 



11. Frustules broadly oval, egg-shaped, or slipper-shaped, the surface of 

 the valve not undulate; costae extending inward from the margin 

 showing prominently Surirella 



PI. 5, Fig. 14 



12. Raphe in hypovalve; pseudoraphe in epivalve; with central and 

 polar nodules in the hypovalve; frustule oval in valve view Cocconeis 



PI. 5, Fig. 12 

 12. Raphe in both epi- and hypovalve; frustules oval or variously shaped 13 



[943] 



