Cone snails are united in a single genus, Conus, because they share a number of derived features, most notably, a feeding structure modified as a harpoon, which the largest species can use to kill fish.
Most snails feed using a radula, which they use for scraping prey off the substrate. To tell which type of structure is ancestral (exhibited by the common ancestor of all snails), and which type is derived (an evolutionary innovation within snails), compare the feeding structure seen in chitons, which is not a snail, but another class of mollusks. They have a scraping radula, which suggests that this is the ancestral feeding structure in snails as well.
To construct phylogenetic trees, many characters, including morphological, developmental, and molecular, especially DNA sequence data, are examined.