There are two main categories of sea shells: gastropods and bivalves. Gastropods have one shell and the creatures that live inside them are slug-like. These shells do not have hinges. The bivalves have hinges and look like identical halves. People find shells all over the world in oceans and areas to where tides can bring the shells. Wetlands come to mind or any swamp or brackish land, where sea water can enter.
Bivalves contain the shells of clams and scallops. Most mussels are in this category. The animals that live in them breathe through holes. Mollusks live inside the shells. Other bivalve mollusks are cockles, soft-shell clams, and bent-nosed clams.
Cockles are either pure white or have drops of tan on their backs. Egg cockles are pure white and Van Hyning's Cockles have the tan droplets on them. Cockles live near the surface in shallow water. Water can easily dislodge them, so they have a foot that allows them to right themselves and move around.
Soft-shell clams live deep in the sandy or mud beach. As they grow, they burrow deeper. Their shells are generally dark brown or black. They have long siphons which they use to get food.
Bent-nose clams have shells with bends in them. The bent-nose moves with its foot across the sand looking for food. It is not a filter-feeder, like most clams, but is more of a scavenger. These shells are easier to find because they do move.
Mussels are blue-black in color. They line rocky shores. When the mollusks are in them, they are very difficult to pry open. Often, single ones lie alone on the beach. Mussels have a whitish inner shell.
Scallops come in a myriad of different colors, ranging from tan to pink. These mollusks can swim away from predators, because they have eyes to see when predators approach. They live on the surface.
Gastropods are a huge class of mollusks with over 35,000 species. These have whorls that spiral up and over the shell point. Again, these come in lots of different colors. The tip is where the mollusk began its life. They all have a tip. Some can reach a length of ten centimeters.
Abalones are well-known gastropods. Their inner shells has tortoise coloring that make them very valuable to collectors. They live on the Pacific coast on rocky shores. The shell can reach nearly four inches.
Source: 1982 The Marine Biology Coloring Book, Coloring Concepts, Inc.