64 DROPS OF WATER. 



dilated ; the prominences are arranged two on 

 each segment. There are more than a dozen 

 species of this genus known. 



In the genus Cosmarium the segments are 

 more or less round, and neither dentated nor 

 rayed at the edge. Cosmarium margaritifermn 

 (Drop I. fig. 4) is rather abundant ; there are 

 more than thirty species known. 



Xanthidium greatly resembles the last, but is 

 armed with spines, and has a circular projection 

 near the centre of each segment. Xanthidium 

 armatum (Drop I. fig. 5) has short thick spines, 

 placed in pairs round the edge of each segment, 

 each spine having several points, and occasionally 

 some scattered on the disc. This species is often 

 found in masses, floating in the water of shallow 



