The morphostructures of larval and juvenile shells of two common species of bivalves of the north-western Sea of Japan, Mytilus trossulus and Crenomytilus grayanus have been examinated. Two types of morphological features were identified on the basis of the results obtained and comparison with materials of other species of the subfamily Mytilinae. One group of morphological features (absence or presence of the nepioconch; form and size of posterior lateral teeth) characterizes the higher taxonomic categories, while the other one (morphological details of general outline of the secondary prodissoconch and juvenile shell; ratio shell height/length; form, size and number of provincular and additional teeth; number of posterior lateral teeth, and others) serves to separate taxa of the generic or specific ranks.
