The term Mollusk Genus refers to four different genera of mollusks, which are Unio (4 letters), Arca (4 letters), Spirula (7 letters), and Trigonia (8 letters). Unio is a genus of freshwater mussels, Arca is a genus of saltwater clams, Spirula is a genus of cephalopods, and Trigonia is a genus of marine bivalves. All four genera are part of the Mollusca phylum, which is one of the largest phyla of animals in the world.