Clam-like shapes can be seen in a variety of taxa from multiple phyla. Below are some of your possibilities.

Clam Larvae

Clams are molluscs, of the phylum Mollusca.

Ostracods

Ostracods are crustaceans, of the phylum Arthropoda.

Cladocerans

Cladocerans are crustaceans, of the phylum Arthropoda.