BY W, WOOLLS. 
1249 
and forms dense masses on the surface of ponds. The writer 
noticed this peculiarity on one occasion in a pond at Richmond, 
but the phenomenon is much more frequent in Europe, where the 
species is said to purify the unwholesome air of marshy places by 
exhaling oxygen during daylight. So far as Australia is con- 
cerned, the order requii’es further observation, but as some of the 
species can only be determined by a powerful microscope, the 
study of them is attended with difficulty. 
