DESHIDS, DIATOMS, AND FRESH-WATER ALGJS. 87 



semi-cell, not divided nor toothed, but often curved ; 

 tubercle a curved row of bead-like elevations. X. 

 antilopceum, Fig. 56. 



Fig. 55. Xanthidium armatum. Fig. 56. Xanthidium autilopseum, 



14. ARTHRODESMUS (Figs. 57, 58). 



1. Spines on the same side curving or spreading from 



each other ; surface smooth. A. incus, Fig. 57". 



2. Spines on the same side curving towards each other ; 



surface smooth. A. convergens, Fig. 58. 



Fig. 57. Arthrodesmus Fig. 58. Arthrodesmus Fig. 59. Spirotsenia con- 

 incus, couvergens. deusata. 



15. SPIROTSENIA. 



Ends rounded ; spiral band closely wound. con- 

 densdta, Fig. 59. 



16. TRIPLOCERAS. 



Surface roughened by small projections arranged in 

 rows around the cell, their tips notched or finely toothed; 

 cell twelve to twenty times as long as broad. T. verti- 

 cilldtum, Fig. 60. 



Fig. CO. Triploceras verticillatuin. 



