1(52 



n 



Genus Docidium Bre"b., 1844 ; em. Lundell, 1871. The cells 

 are comparatively small, elongate, straight and 

 slightly constricted. They are subcylindrical 

 with an inflation on each side of the constric- 

 tion, or nodulose from pole to pole. The base 

 of each semicell is plicated and there is a 

 small basal granule under each plication. The 

 basal plication of the semicells is the principal 

 point of distinction between this genus and 

 Pleurotcenium. The apices of the cells are 

 always truncate and smooth. There is a central 

 chloroplast in each semicell, very irregular in 

 form, and containing an axile row of pyrenoids. 

 The zygospores are unknown. 



There are three British species, of which D. Baculum 

 Breb. is the only one that is widely distributed, and 

 even it is distinctly rare; length 167 262 p.; breadth 

 at basal inflation of semicells 12 13 /* ; fig. 57 A C. 

 D. undulatum Bail, is restricted to the western 

 districts of Ireland and Scotland, and the semicells 

 have a nodulose appearance caused by repeated shallow 

 constrictions from base to apex ; length 187 262 /* ; 

 breadth at basal inflation of semicells 13 16 p, ; fig. 57 

 D and E. 



Fig. 57. A C,Do- 

 cidiumBaculumBr&b., 

 from Bowness, West- 

 moreland; A, x 315; 

 B, base of semicell, 

 x 630 ; C, basal view 

 of semicell, x 630. 

 D and E, D. undu- 

 latum Bail. , from 

 near Oughterard, Gal- 

 way, Ireland ; D, 

 x 315 ; E, base of 

 semicell, x 630. 



Genus Pleurotaenium Nag., 1849. The 

 cells are elongate, straight, and slightly con- 

 stricted. They are cylindrical, with or without 

 inflations on either side of the constriction, or 

 they may be nodulose along their entire length. 

 They are slightly attenuated towards each pole and the apices 

 are usually truncate, often being furnished with a ring of conical, 

 tooth-like projections. There is no basal plication of the semi- 

 cells. The chloroplasts are disposed as longitudinal, parietal bands, 

 several in each semicell, and are furnished with a number of 

 pyrenoids. Sometimes they become broken up into numerous 

 parietal pieces, each containing a single pyrenoid. In most of the 

 species the central portions of the cell contain large vacuoles and 

 occasionally numerous moving granules are observed in the terminal 

 or subterminal ones. These granules are of a yellow colour and 

 black when seen in mass, differing in this respect from the moving 

 granules normally present in the genus Closterium. They are 



