Order Pterioida contains the shells with two unequal valves that have a polished inside surface.

Bivalves and Clams (Class Bivalvia) are found in both fresh and salt water. Several different methods of classification have been proposed for dividing this class into orders and families. The Field Guide to Seashells of the World by Gert Lindner is a good summary of the various taxonomic systems.

Mollusks (Phylum Mollusca) are a large and varied group of animals. Some species are found on land; others are found in freshwater; many are found in the oceans of the world. The most familiar mollusks have an external shell surrounding a soft body. Well-known examples of mollusks are snails and clams. The major classes are as follows:
  
   Ablacophora:                  Worm appearing
   Polhyplacophora (Amphineura): Chitons
   Scaphopoda:                   Tusk Shells
   Gastropoda:                   Gastropods
   Bivalvia (Pelecypoda):        Clams, etcetera
   Cephalopoda:                  Octopus, Nautilus
   Monoplacophora:               A recent discovery

Animal Kingdom contains numerous organisms that feed on other animals or plants. Included in the animal kingdom are the lower marine invertebrates such as sponges and corals, the jointed legged animals such as insects and spiders, and the backboned animals such as fish, amphibians, reptiles, birds, and mammals.