The common marsh periwinkle Littorina irrorata is a typical inhabitant of intertidal marshes and rocky areas. The snails play an integral part in these habitats as detritivores and prey, therefore, their existence is an important link in the food web as well as the health of the intertidal zones. However, various characteristics of their environment influence their population density, distribution, and growth. This book investigates the first official observation of differences in shell length and mass between periwinkles found in rocky and marsh intertidal environments in Eastern Virginia. Factors which most greatly impact the snails were investigated through the implementation of a habitat survey, laboratory trials, and reciprocal transfers.