204 



TetrasporinesB 



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

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

 Lagerheim, 1882 ; Reinschiella De Toni, 1889 [ = Clostridium Reinsch, 1888]; Kirch neriella 

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

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

 Schmidle, 1905 ; Scenedesmus Meyen, 1829 ; Dacti/lococcus Niigeli, 1849 (in part) 

 [= Ouracoccus Grobety, 1909]; Tetradesmm Smith, 1913. 



dat. All x 520. 



Sub-family CRUCIGENIE.E. The Algtr 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 Selenastrese. 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 C. Tetrapedia, 

 fig. 132 F) colonies of considerable size (128 

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



angularin (Nag.) Gay. D ami muc ji a a e of the walls of the mother-cells of 

 E, C. quadrant Morren. 1< , C. . 



Tetrapedia (Kirchn.) W. & G. S. successive generations. In other species ot 



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 C. rect- 

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

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

 without a pyrenoid. In C. 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 crenobium, 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 Kiitzing, 1849 ; Lemmer- 

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

 Hofiiiannia 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 



