Figure 15 The seaweed Fucus replaced by the sea mussel 

 Mytilus on a rock face exposed to wave action, Nahant, Mass. 

 (Courtesy Wisconsin Nat. Hist. Soc.) 



Figure 16 Alga-covered rock face, Nahant, Mass. No Fucus 

 except where tide spurts through a narrow fissure (/) ; Asco- 

 phylium (a) on the protected surfaces; above which is the barna- 

 cle (Balamts) zone (b). (Courtesy Wisconsin Nat. Hist. Soc.) 



