DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 83 



a. Cell triangular in end- 

 view, the angles pro- 

 longed as narrow arms, 

 the ends of which are 

 three- toothed ; surface 

 roughened. St. grdcile, 



Fig- 53- 

 a. Cell with six or seven 



Fi&- 53- — Staurdstrum grdcile. 



Fig. 54. — Staurdstrum macr6cerum. 



St. 



radiating arms, their ends three-toothed. 

 macrdcerum, Fig. 54. 



13. Xanthidium (Figs. 55, 56). 

 The cells bear near both ends a prominence or tu- 

 bercle which may be rounded and smooth, truncate, 

 or apparently encircled by small beads. 

 I. Cell about twice as long as broad; spines short, 

 their ends irregularly toothed ; tubercles circular, 

 beaded. This is the only species with toothed 

 spines. X. armdtum, Fig. 55. 



\ 



Fig. 55.— Xanthidium 

 armitum. 



Fig^ .56. — Xanthidium 

 antilopaeum. 



Fig. 57. — Arthrodes- 

 mus incus. 



2. Cell not twice as long as wide, each half-cell some- 

 what kidney-shaped; spines in four or six pairs 



