Chroococcacece 349 



All the species are rare, but T. Reinschiana Arch. (diam. of cells 5*5 6 p.; 

 fig. 162 D) and T. glaucescens (Wittr.) Boldt occur in lakes and bo^- pools. 

 T. setigera Arch, is a beautiful little species known from X. W. Scotland and 

 W. Ireland, and which has been erroneously referred by some authors to the 

 chlorophyceous genus Tetraedron. The cell-contents of Tetrapedia s<.'ti;f<-i->r 

 are homogeneous and of a pale blue-green colour. 



Genus Coelosphaerium Nag., 1849. The cells are more or less 

 closely grouped to form a hollow, spherical colony. They are em- 

 bedded in a mass of mucus and are arranged just within the 

 periphery of the spherical mass. In form they are globose, ellip- 

 soidal, or ovoidal, and the cell-contents are granular, with so-called 

 gas-vacuoles. 



The genus is a common constituent of the freshwater plankton, and is 

 also found frequently in large ponds. C. Kutzingianum Nag. (diam. of colony 

 48 90/ij diam. of cells 3 3'8 /*; fig. 163 A) is the most abundant species, 

 although C. Niigelianum linger and C. minutissimum Lemm. both occur in 

 quantity in the British freshwater plankton. 



^"A- 



-o, 



oo9 



Fig. 163. A, pcelospJusrium Kiitzingianum Nag., from the plankton of Lough 

 Neagh, Ireland. B, Gomphosphieria aponina Kiitz., from Kestou Common, 

 Kent (x350). 



Genus Gomphosphaeria Kiitz., 1836. In this genus the 

 colonies contain fewer and more scattered cells, which are disposed 

 chiefly towards the periphery of a globular or ellipsoid mass of 

 mucus. The cells are grouped in pairs and are distinctly pyriform 

 in shape. Schmidle has observed the formation of ' microgonidia.' 

 The entire colony is solid and the cells divide alternately in three 

 directions. 



G. aponina Kiitz. (diam. of colonies 64 78 p; length of cells 8'"> 11'5/u; 

 fig. 163 B) is not uncommon in ponds, lakes, and stagnant ditches. 



Genus Microcystis Kiitz., 1833. [Polycystis Kiitz., 1845; 

 Clatkrocystis Henfrey, 1856.] The cells are small, mostly globose, 



