OF THE DESMIDIE^. 



719 



is the third. And this, I may observe, is the order of development of the sub- 

 divisions in the growing segment of the typical Micrasterias : the new seg- 

 ment is first hemispherical; the third subdivision is then developed; and 

 afterwards the first and second are separated. 



" For the purposes of description these three subdivisions might be denoted 

 by the letters «, h, c, and their partial or complete development marked as 

 follows : — When the subdivisions are distinctly separated, their symbols might 

 be separated by commas, thus, a, h, c ; when any two or more are merely 

 marked by a sinuosity, they may be represented thus, a^^h ; and if there is 

 no trace of separation, thus, ah ; and if, at the same time, the direction of the 

 lines separating the subdi\dsions were noted, the full description as regards 

 the divisions of the segments would be given. Thus — [See page 721.] 



CONSPECTUS OF THE GENERA. 



/Joints many times 

 longer than 



broad ; neither 

 constricted nor 

 with lateral teeth 

 or projections. 



-2 ,^ 



-2'Sd 



Joints mostly 

 broader than 

 long, seldom 

 slightly longer 

 than broad; more 

 or less con- 

 stricted, or with 

 lateral teeth or 

 angles, or other- 

 wise figured 



f^ Filament not at- 

 tached 



Filament attached. 



Filament cylindri- 

 cal or subcylin- 

 drical 



/^Joints not con- 

 stricted . . . 



s 



Joints more or 

 less deeply 

 constricted . 



''Endochrome arranged 



in spiral bands GtENIcularia. 



Endochrome a simple 

 central longitudinal 

 contracted band . . . Gonatozygon, 



Endochrome a single 

 longitudinal flatten- 

 ed band Leptocystinema. 



^Joints constricted or 

 with a projecting an- 

 nular rim at one or 

 both ends Hyalotheca. 



Joints with a bidentate 

 process or angle at 

 opposite sides Didymoprium. 



Filament 3-4-angu- 

 lar ; joints having 

 the external margin 

 plane or slightly cre- 

 nated, united to each 

 other by projections 

 springing from the 

 outer portion of each 

 extremity, thus pro- 

 ducing intervening 

 central foramina ... Aptogonum. 

 ( Filament 3-4-angu- 

 lar or compressed ; 

 joints either closely 

 united by a thick- 

 ened border for their 

 entire end-margin, 

 or by projections 

 producing interven- 

 ing central foramina, 



as in last Desaiidium. 



Filament compressed; 

 joints united to each 

 other by minute tu- 

 bercles or gland- 

 like processes SriiiEROzosMA. 



Filament compressed 

 or 3-angular ; joints 

 without intermedi- 

 ate tubercles or pro- 



Spondylosium. 



