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Omastar is a sky-blue Ammonoidea Pokémon, with several tentacles and a pale yellow, helix spiraled shell on its back. Its shell constantly grows, getting heavier over time and thus limiting its mobility. It is the weight of the shell that is most often cited as the cause for Omastar's extinction. Several sharp spikes line the shell's midsection, spanning from its forefront to just a little towards its back. The shell's hem has two circular indents where Omastar's eyes come out. Its eyes have yellow sclera, with vertically-slit pupils. It has an oval mouth with a four-way jawed, fang-like beak. Once caught, it never lets go of prey, immediately biting down on its victim with immense power.
Before its own shell led to its extinction, Omastar swam the seas in search of prey. Even the hard shells of prehistoric Shellder stood no match to Omastar's deadly powerful bite, and were quickly made a meal of by having their insides sucked out.