Instruct students to examine the shells, observing texture, shape, pattern and structure. They should come up with any system of grouping the shells they like, and classify the shells. After they have them classified, they should give a name to each category, and write down a description of the features of each category. Make each student in the group take notes, so all are on task. Discuss with the entire class the categories which emerged, asking students to and describe their categories.
Many groups are likely to have similar categories, perhaps with different names.
Explain that people have observed these categories for a long time, and scientists have tried to agree on a common system of classification, so they can use the same terms to describe the same animals. Hand out copies of Golden Guide to Shells, and have students turn to the overview of univalves, on page 4 and 5, which contains most of the groups they are likely to find. Ask them to see if they find any matches or overlaps with the categories they created. Ask them to add, whenever possible, the scientific name for the category to their notes. Their original classification system is legitimate, but they should understand the value of everyone agreeing on a common system.