48 Fresh-water Biology 



{Continued from page 46) 



8— Cells lobed or incised. CeUs mostly oblong or elliptical moderately lobed (figs. 24 

 26), Euastrum; cells very much compressed, deeply lobed or mcised (fig. 15, 18) 



JViicrHstcnss 



—Ceils with a more or iess entire margin, often furnished with warts or spines (figs. 21, 

 22) Cosmarium; with spines commonly in pairs, Xanthidmm; spmes 4 or 8 occa- 

 sionally 16 Arthrodesmus 



9— Colonies spheroidal, joined by gelatinous bands. Bands narrow, few cells, Cosmo- 



cladlum; bands very broad, many cells ... .Uocardium 



—Colonies thread like; ceUs attached by their apices to form long filaments lU 



10— The Une of division of the cell where the new and old parts of the ceU waU are fitted 

 together, does not develop a girdle during division. ••••.•;•.••••••■;•••••••••• |^ 



—The line of division of the cell develops a girdle durmg division. Cells short, rarely 



circular with produced angles (fig. 9), Desmidium; cells elongate, cylindrical 



Gynmozygas 



11— Cells attached by special apical processes. Apical processes very short, Sphaero- 

 zosoma; apical processes long and overlapping the apices of the adjacent cells. . . . 



^ ^ Onychonema 



Ceils slightly constricted, Hyalotheca^ cells deeply 

 constricted , 



—Apices of cells plane and flat. CeUs slightly constricted, nyaiotneca; ceiis c _ 

 ^-^^cfrirfpH Spondylosium 



BACILLARIACEAE (Diatoms.) 



* 



1— Cells in transection circular, less commonly polygonal or elliptical and rarely ir- 

 regular; valves marked concentrically or radiately by dots, areolations, lines^r 



ribs; cells often with spines, processes or horns. • • -^ - • Cetricaea I 



—Cells in transection narrowly elliptical to linear, less commonly broadly elliptical, 

 lunate cuneate or irregular valves marked pinnately or transversely by dots, areo- 

 lations lines or ribs; cells without spines Pennatae— i) 



2— Cells short box-shaped or discoid, mostly circular in transection, usually without 



horns or projections 



— Cells of other forms 



3— Cells forming filaments, girdle side marked. Valve uniformly marked (figs. 1, 2, 3 4) 

 ( = Lyslgonium), Melosira; margin and center of valve differently marked, central 



portion areolated .••••>.• -n" Vn\- " r ' i /u >? 



-Cells single, girdle side not marked. Without spmes (figs. 9, 10). Cyclotella; with 



circle of spines (figs. 5, 8) Stephanodiscus 



4— Cells two to many times as long as broad, circular, rarely round elliptical in transec- 

 tion; girdle with numerous interzones • Khizosolenia 



—Cells box shaped, as long as broad or shorter, elliptical, sometimes lunate in transec- 

 tion; valves with horns, valves with transverse septa, without spines. . . lerpsinoe 



5— Rachis of the valves (i.e. the line between the divergent pinnate markings) evident as 

 a narrow unmarked strip (pseudoraphe), rarely wanting; valve without slit (raphe^ 



text fig. 2) ; c, 



—Rachis otherwise; raphe (slit) present ■^•^ 



6— Cells usuaUy but little shorter than broad, or longer, with numerous interzones 



mostly united into filaments • • ;••,••,• '■' "c" 'a 



—Cells prevailingly much shorter than broad (rod-shaped, the longer axis of rod repre- 

 senting transverse axis of cell) often united into filaments » 



7— Transverse rib of valves, when present, not extending into the cell cavity. Valves 

 with a few prominent transverse ribs, Tetracyclus; valves finely transversely 



striate only, pseudoraphe absent (figs. 28, 36) (=Striatella) Tabellana 



—Transverse ribs of the valves extending deep into cell Denticula 



* Adapted from Bessey. 



