Complex sentence: The amethyst gem clam, "Gemma gemma", is species of very small saltwater clam, a marine bivalve mollusk in the family Veneridae, the Venus clams.
Split into simpler sentences: The amethyst gem clam, "Gemma gemma", is a small species of marine bivalve mollusk. It is a member of the family Veneridae, the Venus clams.