274 Dr. G. C. Wallich on Desmidiacea from Lower Bengal. 



can 1)e more incongruous than the sequence, immediately after 

 Spharozosma, of a genus containing lenticular- shaped, multi- 

 lobed fronds, as is the case in Micrasterias ; whilst genera with 

 simple, cylindrical, or subcylindrical fronds (namely, Teimemorus, 

 Ducidium, Penium, Clvsterimn) hold a place directly following on 

 complex angular forms, such as are included under Staurastrum 

 and Didymachdon. 



The occurrence of these isolated filamentous species shows 

 that, however valid may be the character derived from it for 

 purposes of specific distinction, it affords no good basis for 

 generic separation, when taken by itself. 



Docidium asperum, which also occurs in the filamentous form 

 in Eengal, has on this ground alone been elevated into a distinct 

 genus by De Bary ; and of such importance is the character 

 generally considered, that some of the most eminent authorities 

 have deemed it necessary thereupon to transfer D. asperum to 

 the true Confervoid Alg?e. Unless they are prepared to transfer 

 the entire genus, the removal of the isolated species must be 

 cancelled; for the Ik'ugal filamentous Docidium is closely allied 

 to D. coronatum, Ehr., and constitutes as distinct a form as D. 

 clavatum or D. Ehrenhergii. I v/ould mention, in order to pre- 

 vent misapprehension on this subject, that the occasional cohe- 

 rence of a few fronds, under favouring circumstances, as is often 

 seen in Cosmarium monilifcrinn and some other species, is quite 

 distinct from the normally filamentous condition to which refe- 

 rence is here made. 



Synopsis of the Genera. 



** Sporangia spinous or tuberculate. Frond distinctly constricted 

 at the junction of the segments. 



8. IIoLOCYSTis. Frond much compressed or lenticular. Segments 



horizontally lobate. 



9. Micrasterias. Frond lenticular or tabular, lobate. Lobes 



dentate, or inciso-dentate. 



10. EuASTRUM. End view of segments oblong or sinuate. Segments 



lobate or sinuate. Lobes emarginate or incised, and gene- 

 rally furnished with inflated granular protuberances. 



11. Cosmarium. Frond simple. Segments entire. 



12. Xanthidium. Segments entire, with symmetrically arranged 



processes or spines. 



13. Staurastrum (including Arthrodesmns). End view com- 



pressed, angular, or radiate. Segments mucronate, spinous, 

 or with dentate projections. 

 11. DiDYMOCLADON. Segments angular, with geminate or alternate 

 dentate projections or spines. 



