6o 
W. B. Crow 
above remarks will have made clear the species of Microcystis can 
only be distinguished definitely when a considerable amount of 
material is available and individual variation is taken into account. 
Moreover, there has been considerable difference of opinion with 
regard to the limitations of most of the species, and some attempt 
has been made to enumerate the essential features of the species met 
with in the Ceylon collections. 
KEY TO THE GENUS MICROCYSTIS. 
Cells spherical. 
z. Cells containing pseudovacuoles. 
a. Cells 3-7/i diam. 
1. Colonial envelope definite, refractive, often stratified. 
i. Colony generally compound. M. viridis 
ii. Colony generally simple. M. marginata 
2. Colonial envelope less definite, colonies compact. 
i. Colony not greatly lobed nor very elongated. 
* Colony clearly clathrate. M. ceruginosa 
* * Colony not clearly clathrate . ... M. flos-aquce 
ii. Colony elongated to many times its diameter, segmented into 
partial colonies. M. pseudofilamentosa 
iii. Colony very greatly lobed, in the form of cuneiform figures. 
* Colony clearly clathrate. M. ochracea 
** Colony not clearly clathrate. M. scripta 
3. Colonial envelope quite indefinite, cells very loosely scattered 
M. protocystis 
( 3 . Cells less than 3^ diam. 
1. Colonies generally compound .... M. ichthyoblabe 
2. Colonies simple, pellicular ....... M. firma 
b. Cells not containing pseudovacuoles. 
a. Cells distinctly yellow. M. fuscolutea 
( 3 . Cells not yellow, mostly pale blue-green. 
1. Cells 5-5-5/i diam. . M. pallida 
2. Cells less than 3/x diam. 
i. Colony elongated to many times its diameter . M. stagnalis 
ii. Colony not greatly elongated. 
* Colony free-floating, clearly clathrate . . M. holsatica 
** Colony free-floating, or attached to stones, not clathrate 
M . pulverea 
*** Colony attached to water-plants M. parasitica 
B. Cells elongated. M. elabens 
The above includes all the fully known freshwater species. For 
other forms consult Tilden(8) and Forti(2). 
