374 Mr. J. Ralfs on the species of Desmidium. 



The mucous sheath is narrower than that of D. mucosum, and 

 in both a portion of it seems to belong to each joint ; in both also 

 it is elastic, which may be the cause why the joints separate so 

 easily. The elasticity is shown by its becoming longer than the 

 joints after their separation. 



I learn from Mr. Berkeley, who kindly sent me a portion from 

 Desmazieres' specimen, that Desmazieres figures the joints as 

 slightly compressed, with a rounded angle both of the filament 

 and sheath on each side, and thus agrees exactly with my own 

 observations. 



Desmazieres, however, represents the endochrome in two parcels, 

 each consisting of five granules. In the specimens I have ex- 

 amined the endochrome has sometimes four and sometimes five 

 rays. 



The filaments are twisted, and consequently at about every 

 tenth joint the angles are very perceptible, whilst in the two 

 central joints they are almost invisible ; on altering slightly the 

 position of the filament the angles in the latter become visible, 

 and in the former disappear. 



On account of the oval form of the joints the angles of each are 

 separated, and thus the filament acquires a pinnatifid appearance ; 

 the notch is sharply defined, as if a triangular bit had been 

 snipped out. 



2. D. mucosum, Breb. Filaments cylindrical, appearing very slightly 

 crenate ; joints generally half as long as broad. PI. VIII. fig. 2. 

 Conf. dissiliens^ng. Bot. t. 2464. (crenature in the figure too deep)'. 

 Cwm Bychan, TV. Borrer, Esq. ; Tunbridge Wells, Mr. Jenner ; 



near Bedgelert and about Dolgelley, N. Wales ; Swansea, S. Wales; 



plentiful near Penzance, Cornwall; High Beach, Essex; Hertford 



Heath and Wormley West end, Herts, Mr.Hassall. 



Filaments slenderer than those either of D. Swartzii or D. cy- 

 lindricum, mucous, adhering firmly to paper, with a broad mucous 

 sheath, cylindrical, without any angles, in which respect it differs 

 from the other three species. Each joint has a shallow groove 

 passing round it (thus resembling a small pulley-wheel) which 

 gives the crenate appearance to the filaments ; the crenatures are 

 generally very shallow, sometimes nearly obsolete, but I have oc- 

 casionally seen them deeper, and thus more like the figure in 

 ' Eng. Bot/ There is scarcely any depression between the joints ; 

 in this circumstance it differs greatly from D. cylindricum. 



If kept in water for a few days it separates into single joints, 

 each joint having a perfect mucous covering ; both the filament 

 and sheath are cylindrical. 



The endochrome generally is disposed in a stellate manner 

 (when seen transversely), having a small central ring and six or 

 seven rays. 



