2. 



DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 8l 



Not widest in the middle, the ends not tapei-ing. 



T. Brebissdnii, Fig. 44. 



10. DociDiuM (Figs. 45, 46). 

 A slight, rounded enlargement on each side of the 



central constriction; smooth; no terminal 



vacuoles. D. Bdculum, Fig. 45. 



c 



D 



Fig. 45.— Docidium Bdculum. 



Fig. 46 — Docidium crenulatura. 



2. Two or several small enlargements on each side 



of the central constriction, giving the margins a 



wavy outline; coarsely punctate; from 8 to 16 



times longer than broad. D. crenuldtum, Fig. 46. 



II. CosMApiuM (Figs. 47, 48, 4q, 50). 



The ends of Cosmarium are never notched nor 



incised. They may be, and often are, rough or warty, 



but the ends are always entire. There are about one 



hundred species. The following are common: 



1. Surface smooth; cell less than twice as long as 



broad, two semi-cells evenly rounded. C. Jidlfsii, 

 Fig. 47- 



2. Surface smooth; cell about twice as long as broad, 



the margins of each semi-cell slightly sloping tow- 

 ard the flattened ends. C. pyramtddtum, Fig. 48. 



Fig 47.— Cosmi- Fig. 48.-Cosm4- Fi.r. 4,.-Cosm4- Fig. jo^Cnsma- 

 * *' - ~ ■ riummargari- num Brebiss6- 



rium Rdlfsii. 



num pyrami- 



tiferum. 



