How to Distinguish from Similar Species: Butter clams Saxidomus gigantea have only concentric sculpture. Native littleneck clams Protothaca staminea are as high as they are long, their ridges are less prominent, their siphons are fused all the way to the tips, and the inner ventral margin of the shell is finely serrated. These other species do not have purple markings inside the shell. Another introduced species, Nuttallia obscurata, has purple markings inside the shell but the light brown periostracum is smooth and shiny.