112 THE AMERICAN MONTHLY [June, 



a. Length 5-12 times the width {b) . 



a. Length more than 12, less than 20 times the width (e). 



a. Length 20 times or more than the width (_/"). 



b. Ends suddenly contracted ; cell fusiform, 5 times longer than wide, smooth, 



iiasittum^ 41. 

 b. Ends not contracted, but tapering, acute ; chlorophyl bands several, 



granules in i row lanceolatuni^ 39. 



h. Ends not contracted, rounded (c). 



b. " " truncate {d) . 



c. Cell slightly curved, small, 6-12 times longer than wide, smooth, 



ac7ittinr^ 44 

 c. " tit*. :^-io times longer than wide, smooth obtustim^ 38 



c. Cell nearly straight, 7-12 times longer than wide, decussately striate, 



dccussatum, 39 



d. Cell slightly curved, 6-12 times longer than wide, striate; vacuole 



distinct didymotocti?n ^ 39 



c. Cytioderm with 4-5 longitudinal striie, often with 2 or three transverse 



bands and decussating stria^ a)igiistatum^ 40 



c. Cytioderm striate ; ends slightly incurved ; glol)ules about 20 in each 



semi-cell, axilary lijieatuvi^ 43 



f. Long (19-24 times longer than wide) ends thin, finely rounded, 



strigosuni^ 42 

 f. Long (20-40 times longer than wide) slightly curved, smooth, or with 



1-4 transverse strife ; diameter ^J-^tt to 5^^^^^ in. (5-13,'/) /'linci'ditju, T)^ \ 



macilentuiu^ 38 ; gracile, 39. 



f. Long (20-40 times longer than wide) slightly curved, smooth, or with 



1-4 transverse striae ; diameter ^^^'otv to ■^-^X)i5 i"- (3~4.") cell acicular, 



szibtile, 158. 



g. Ends inclined upward at a dorsal depression ; ventrum slightly concave ; 



strice fine, numerous titrgidi(m^ 41. 



g. Ends suddenly contracted to a narrow point; cell slightly curved, 



atteiiuatu})!^ 41 . 

 g. Ends not suddenly contracted (/^). 



h. Cytioderm deeply striate, distinctly granulate or areolate areolattim, 43. 

 //. '' indistinctly striate; cell linear fusiform, 15-24 times longer 



than wide acerosttm^ 41. 



/i. Cytioderm indistinctly striate ; cell semi-lunar, ^-6 times longer than 



wide L7t)iula^ 40. 



/. Diameter ^-^ to ^\-^ in. (75 to 1 10 />.) ; cytioderm smooth, EhrenbergiL 45. 

 /. Diameter j-J-jj to y^r '^^'^' (40-60 ,a) ; globules a single row, 



7noniUferiim^ 45 ; Leiblciuii ^ 46. 



/. " •' ^^ cell curved, rapidly tapering into 



narrow, somewhat upwardl\'-curved ends ; cell 6-'$> times longer than 



wide Ralfsii^ 46. 



y. Cytioderm with many distinct stris ; length 6-16 times the width; 



vacuole large striolatum, 42. 



j. Cytioderm with fine striie ; length 13-16 times the width ; vacuole small, 



deco>'utHj 43. 

 J. Cytioderm with 5-8 distinct striae ; length 6-S times the width ; vacuole 



large costafifm^ 42. 



J. Cytioderm smooth; cell crescent-shaped, often subsemicircular {k) . 

 J. •' " cell not conspicuously ci-escent-shaped (/). 



k. Ends separated 7-10 times the diameter ; width y^Vo" ^o yru '^^^- ( ^ 6-20 ,a) , 



Diancs^ 44. 



