Either littleneck or top neck clams (both hardshell clams) work well in this recipe. Larger clams, such as cherrystone or chowder clams, tend to be chewy and tough and are best chopped before using.   When buying fresh clams, make sure the clams do not have cracked or broken shells and ensure the clam shells are tightly closed. Give any open-shelled clams a tap, and if they don't shut, discard the clam. (Open clams or clams with damaged shells are indicators of dead clams, which you should never eat.) Fresh clams should smell like the sea, not fishy.