204 



Tetrasporinex 



The geneva are : Ankistrodesmus Corda, 1838 ; em. Ralfs, 1848 [inclus. Rhaphidium 

 Ktitzing, 1845 ; Schroderia Lemmermann, 1898] ; Selenastrum Reinsch, 1867 ; Actinastrum 

 Lagerheim, 1882 ; Reinschiella De Toni, 1889 [ = Clostrid ium Reinsch, 1888]; Kirchneriella 

 Scbmidle, 1893 [inclus. Selenoderina Bohlin, 1897] ; Closteriopsis Lemmermann, 1898 ; 

 Lauterborniella Schmidle, 1900 ; Desmatractum W. & G. S. West, 1902 ; Didymogenes 

 Schmidle, 1905 ; Scenedesmus Meyen, 1829 ; Dactylococcus Niigeli, 1849 (in part) 

 [=0uracoccus Grobe"ty, 1909]; Trtradesmus Smith, 1913. 



Sub-family CRUCIGENIE^E. The Algre of this small group consist of 



colonies of few or many cells arranged with 

 much regularity in the form of a flat plate. 

 The cells vary much in outward form, but are 

 mostly somewhat rounded and never elongate 

 as in the Selenastreae. Four autospores arise 

 in each mother-cell. They are disposed in one 

 plane, and in certain species of Crucigenia (C. 

 rectangularis, fig. 132 A, and G. Tetrapedia, 

 fig. 132 F) colonies of considerable size (128 



H 



angvlaris (Nag.) Gay. D and 



E, C. quadrata Morren. F, C. 



Tetrapedia (Kirchu.) W. & G. S. 



dat. All x 520. 



Fig. 132. AC, Crucigenia rect- cells) may be formed by the conversion into 



muc ii a g e of the walls of the mother-cells of 



. 



successive generations. In other species of 

 Crucigenia (C. quadrata, C. Lauterbornii, etc.) 

 the colonies never become very large, and con- 

 sist mostly of from 4 to 16 cells. Even in the 



large colonies the cells remain in distinct groups of four, and there is always 

 a quadrate or rhomboidal space in the centre of each group, small in G. rect- 

 angularis, G. quadrata, etc., but large in C. Lauterbornii and G. fenestrata. 

 Each cell contains a single parietal chloroplast, often massive, and with or 

 without a pyrenoid. In G. irregularis, which is known from the lakes of 

 Norway and the Shetlands, ^the colonies are large and gradually become 

 irregular ; in the Madagascar species, C. emarginata, the cells are emarginate 

 at each pole ; and in C. appendiculata the wall of the old mother-cell is 

 retained in four pieces which have the appearance of appendages, one piece 

 being attached to the outer margin of each of the four daughter-cells. 



In Tetrastrum the colony is always a four-celled ccenobium, each cell 

 being furnished on its outer margin with from two to five spines (fig. 132 G 

 and H). 



The genera are : Crucigenia Morren, 1830 [inclus. Staurogenia Kutzing, 1849 ; Lemmer- 

 mannia Chodat, 1899 ; Willea Schmidle, 1900 ; Crucigeniella Lemmermann, 1900 ; 

 Hofmannia Chodat, 1900]. Tetrastrum Chodat, 1895 [= Cohniella Schroder, 1897]. The 

 various species occur in the quiet waters of ponds and boggy pools, and some are found in 

 the benthos and plankton of lakes. 



