200 PROCEEDINGS OF SOCIETIES. 



not in the manner characteristic of the genus Staurastrum at 

 large, that is, forming vertical plates radiating from a central 

 axis. Mr. Archer had ere now drawn the Club's attention to the 

 fact that the chlorophyll-contents in Si. tumidum, for instance, 

 are deposited iu rather broad, irregularly margined bands upon 

 the interior of the cell-wall, that the same holds good in the spe- 

 cies of Xanthidium, but the " bands" are there fewer and broader, 

 forming, in fact, in each segment, four parietal " quarters" 

 (rather than " bands"), these interrupted or separated by as many 

 narrow, nearly straight vacancies, running down the centre of 

 each front and each lateral aspect of the segment. This is a 

 character of Xanthidium, apparently not generally recognised, 

 and would seem to tend still further to cast a doubt on the 

 validity of Pleurotsenium as a genus. In his fine work on the 

 Conjugatae (' Untersuchungen iiber die Familie der Conjugaten,' 

 pag. 72), Professor de Bary, in referring to Xanthidium, speaks 

 of the chlorophyll-contents as radiate (" strahlig"), but as "not 

 yet more exactly known ;" however, this is an error, for the chlo- 

 rophyll-layers really line the interior of the cell-wall. Now, the 

 presence of the conspicuous chamber or cavity, vpith "dancing" 

 granules close to each end of certain forms, referred by authors 

 to the genus Pleurotaenium (Niig.), would actually seem to be the 

 only internal characteristic that might be considered as wanting 

 to make such forms as those alluded to here fall under that genus, 

 ■whilst, indeed, in some forms referred by authors thereto, this 

 character is absent ; nor, indeed, does it form one of those enter- 

 ing into Nfigeli's diagnosis oi\m, genus. In other w^ords, certain 

 species (e. g. Staurastrum longispinum. Bail., St. tumidum, Breb., 

 Cosmarium turgidum, Breb., Cos. de Baryi (de By.), Arch., 2lan- 

 thidium armatum, ^veb., 2!. fasciculatum, Ehr., &c.), if we confine 

 our attention to their internal characters, offer the main feature 

 claimed by Niigeli for his genus Pleurotaenium (' Glattungen einzel- 

 liger Algen,' pag. 104), but if we, on the other hand, have regard 

 to their external characters, these heterogeneous forms naturally 

 relegate themselves to several diflerent and well distinguished 

 genera ; nay, as Mr. Archer had ere now drawn attention to, not all 

 the species referable by external figure to Docidium (Breb.) agree 

 in the parietal chlorophyll-bands. What, then, can we do with 

 such a form as Staurastrum longisjnnum (Bail.), now exhibited, but 

 leave it still a Staurastrum? Nor are we, seemingly, as Mr. 

 Archer previously ventured to suggest, as jQtripe enough for the 

 genus Pleurota3uium. Perhaps the identification of the present 

 form with Bailey's figure (now exhibited to the Club) may be 

 thought to be somewhat shaken, by his neither showing any 

 delineation whatever of a similar arrangement of the contents, 

 nor making any allusion or throwing out any clue thereto in the 

 text. Still, Mr. Archer thought he could trace just a faint ap- 

 pearance in Bailey's figures of the "bands" being indicated by 

 the rough shading there to be seen, which may indeed, perhaps, 

 be iust as much accidental as intended. The form now shown to 



