Several morphological characteristics of the shell in cockles (genus Cerastoderma) were investigated. A significant inverse relationship between rib number and shell curve was revealed in Cerastoderma glaucum. Morphometric and meristic descriptors were correlated with some environmental factors, such as salinity and temperature. Analysis of allometry in C. glaucum and Cerastoderma edule revealed significant differences between the two species. The results are discussed from an adaptive perspective, providing possible hypotheses on the steps which might have driven the evolution of form of the studied species during the process of adaptation to coastal tidal and confined habitats.