210 



OUR COMMON BRITISH FOSSILS. 



Others are largely chitinous, as the common 

 sea-mats (Flustra], whose seaweed-like fronds often 

 appear as algae in seaside albums, but which are 

 really an innumerable colony of little cells placed 

 back to back. 



Fig. 166. Recent Polyzoon (Membranipora), encrusting Sea-weed. 



Stones, rocks, oyster and other shells are fre- 

 quently invested with the spreading growth of these 

 lovely organisms. And in this respect we find their 

 habits have not altered since the Silurian period, for 



