128 BEITISH DESMIDIAOE^B. 



Division II. Cell-wall granulate, verrucose, or papillate. 



[As in Division I (the smooth section of the genus) the arrange- 

 ment of the species has been based on external form. We stated 

 in vol. ii, p. 127, that this arrangement was an artificial one, and 

 while admitting that it has the appearance of being somewhat 

 arbitrary, the facilities it affords for the grouping of closely allied 

 species cannot be over-estimated. We are gradually arriving at 

 the conviction that external form is the dominating factor in the 

 determination of the species-groups in Desmids ; and this being the 

 case, much of the suggested artificiality of the present arrangement 

 of Cosmaria disappears. In. all large genera the arrangement of 

 species in sequence can never more than approximately represent 

 their relationship.] 



SECTION A. Semicells semicircular, subsemicircular, or semi- 

 elliptical in outline ; cells approximately circular, subcircular, or 

 elliptical in outline. 



* Constriction deep. 

 f Sinus open. 



+ Semicells with a basal tooth, granules confined to 

 the margin. 



136. C. quadridentatum. 



++ Semicells without a basal tooth, uniformly granu- 

 late. 137. C. Corriense. 



138. C. Simii. 

 ft Sinus linear. 



Margin of semicells crenate. 



139. C. c&latum. 

 + Margin of semicells entire. 



Cells granulate. 



140. C. radiosum. 



141. C. intermedium. 



Cells with marginal papilliform granules. 



142. C. cristatum. 



Cells with marginal series of emarginate 

 verrucse. 143. C. monomazum. 



144. C. quadrifarium. 

 ** Constriction not very deep. 



f Sinus acute ; margin of semicells crenate. 



145. C. dovrense. 



ft Sinus a wide excavation ; margin of semicells entire ; 

 cells granulate. 146. C. isthmium. 



147. C. excavatum. 



SECTION B. Semicells circular or subcircular in outline. 



* Constriction slight. 148. C. subexcavatum. 



** Constriction fairly deep. 149. C. orbiculatum. 



150. C. prtecjrande. 



