rig^Jre 12 Siphonaria glgas on rocks near high v/ater level. 



Giilf of Panama. 



\, 



Figure 13 — 



Snail basins filled v/ith sea water on the top of rocks in 

 the tidal zone give shelter to thousands of small gastropods 

 and crahs. Bottom is freo.uently covered Tsy a green carpet 

 of Halicystis. 



2U 



