The marine gastropod, Siphonaria pectinata, has an active part in the complex formation of secondary solution features characteristic of outcroppings of beach rock in south Florida. Analyses were made of the distribution, anatomy, and feeding activity ofthis snail. The results show that these molluscs are concentrated in areas of maximum algal growth. While grazing they rasp substrata presoftened by algae so that new rock surfaces are continually exposed to biochemical erosion.