My uncle calls clams and such molluscs. Lobsters, crabs, shrimp, crawfish etc are shellfish. At least according to him.
Your uncle is mostly correct.
Clams, oysters, snails, and squid are all members of the phylum
Mollusca. Lobsters, crabs, crayfish, and shrimp are members of the phylum
Arthropoda, and the subphylum
Crustacea.
The USDA and FDA define "shellfish" to include molluscs and crustaceans.