720 



SYSTEMATIC HISTOKY OF THE INFUSOEIA. 



Frond often as 

 broad as long, 

 rarely, if ever, as 

 much as three 

 times longer than 

 broad. Sporan- 

 gia mostly orbi- 

 cular and spi- 

 nous, rarely orbi- 

 cular or quadrate 

 and naked 



Fronds deeply con- 

 stricted ; seg- 

 ments more or 

 less deeply lobed, 

 or if merely un- 

 dulate or taper- 

 ing, the ends 

 acutely notched.. 



^ Segments 3-lobed, lateral 

 lobes attenuated, their 

 apices entire or bifid. . . Tetrachastrum. 



Segments 3-5-lobed, la- 

 teral lobes expanded, 

 incised, their external 

 margins dentate or 

 rarely sinuate Micrasterus. 



Segments 3-5-lobed, or 

 sometimes only late- 

 rally emarginate or si- 

 ■{ nuate, undulate or 

 tapering ; lateral lobes 

 rounded, entire, or si- 

 nuately emarginate ; 

 end-lobe mostly cen- 

 trally emarginate or 

 concave, the segments 

 with variously disposed 

 inflated circular promi- 

 nences (the two latter 

 characters never simid- 

 taneously absent*) ... Euastrum. 



Segments not lobed, en- 

 tire, mostly rounded, 

 rarely undiilate at the 

 margin, ends never 

 emarginate, sometimes 

 with a solitary central 

 inflated prominence on 

 each front surface ; 

 without spines or pro- 



Fronds distinctly, 

 mostly deeply, 

 constricted ; seg- 

 ments mostly en- 

 tire, or if some- 

 what undulate, 

 the ends not 

 notched 



COSMARIUM. 



Frond mostly many 

 times, rarely less 

 than three times, 

 longer than 



broad. Sporangia 

 smooth {Penium 

 annulatum and 

 SpirotcBnia mus- 

 cicola are some- 

 times not more 

 than twice as 

 long as broad)... 



f Frond dis- 

 tinctly con- 

 stricted at 

 the middle. 



Frond either not at 

 all constricted, or 

 with a slight and 

 gradual attenua- 

 tion towards the 

 middle 



Segments compressed, 

 entire, spinous, with a 

 central circular, cylin- 

 drical, or conical pro- 

 jection on both front 

 surfaces Xanthidium. 



Segments compressed, en- 

 tire, either with two or 

 with four acute teeth 

 or simple or geminate 

 subulate spines placed 

 on the external angles 

 or prominences, with- 

 out a central projec- 

 tion Arthrodesmus. 



Segments m e. v. angular 

 or radiate Staurastrum. 



^ in- r Ends trilobed. Triploceras. 



lated at the \ 



base [ Ends truncate. Docidium. 



Segments not 

 inflated at 



the base ... Ends notched. Tetmemorus. 

 /^ Frond curved or arcuate, 



not constricted Closterium. 



Frond straight, ends 

 truncate or rounded, 

 scarcely or not at all 



constricted Penium. 



Frond sti'aight or nearly 

 80, endochrome spi- 

 rally twisted Spirot^nia. 



y 



* Euastrum crenatim (Kg.) is perhaps an exception. 



