22 Bulletin Wisconsin Natural History Society [Vol. 11, Nos. 1-2 
a greater variety of species and more individuals occurred at 
Stations 7, 8 and 9 (fig. 1) than elsewhere. Exposure to wind 
causes the algae to give place to Mytilus and Balanus — -which 
are better suited to stand the beating of the waves (figs. 12, 13, 
14, Stations 2, 9 and 8). Sunlight is of course necessary for the 
growth of algae and at Station 1, where vertical cliffs have a 
northern exposure, the Ascophyllum zone is nearly absent. The 
effect of the slope of the rock is bound up somewhat with the 
character of its surface, but in general the more nearly vertical 
Fig. 17 . Showing the Abundance of Littorina litorea and Balanus on a 
Rock when Boulders are Present. 
a cliff*, the more abundant is its fauna in individuals and the 
more limited in species. At Station 2 a gentle smooth slope was 
exposed to the full sweep of the waves and was nearly bare; 
whereas a rough beach near it with a similar slope and exposure 
was completely covered with Balanus, Mytilus, Chondrus, Fucus, 
etc. Boulders or crevices greatly increase the abundance of the 
fauna (fig. 17) and cause all the zones to migrate upward. Expos- 
ure to the ocean also causes all the zones to seek a higher level 
(fig. 18). 
