72 



AQUATIC MICROSCOPY FOR BEGINNERS. ' 



h. Arms none; spines four, two on each end. Ar- 

 throde'smus, 14. 



h. Arms none; spines several, on the edges; a round- 

 ed, truncate, or denticulate tubercle in the 

 center of each semi-cell. Xahthidium, 13. 



i. Chlorophyl in a spiral band, cell cylindrical. Spi- 

 rotcenia, 15. 



i. Chlorophyl not in a spiral band, cell cylindrical 

 ik). 



k. Surface roughened by oblong often tooth-like 

 elevations. Triploceras, 16. 



k. Surface smooth, ends rounded, neither divided 

 nor notched. Pdnium, 17. 



L. End view with from three to six or more angles 

 (ot). 



/. End view not angular, margins smooth, dentate, 

 or crenate, without spines; ends always entire. 

 Cosmdrium, i^i. 



rn. Angles obtuse, acute, or with horn-like prolonga- 

 tions. Staurdstru7n, 12. 



I. Didym6prium. 



Each cell in the band longer than broad ; two round- 

 ed or angular teeth on each narrow end ; case or sheath 

 distinct, colorless. D. Grevillii, Fig. 17. 



Fig. 17. — I)idym6prium Grevillii; Fig. i8. — Sph2eroz6sma pulchrum. 



2. SpH/Broz6sma. 

 Each 'Cell in the band about twice as long as broad. 



