Cover fossil

Cover Fossil is a fossil in Pokémon Black and White that can be resurrected to Tirtouga.

It looks like a shell of a tortoise. The player can get the Cover Fossil, or the Plume Fossil, at Relic Castle. A female Backpacker will have them, and says that she can't hold them all, giving you one generously

Games
This fossil is exclusive to Pokémon Pearl and Pokémon Sun and Moon. The Armor Fossil is the only fossil available until the player receives the National Pokedéx. They can rarely be found in the Underground through mining, and can be revived at the Oreburgh Mining Museum to form the Pokémon Shieldon. In Pokémon HeartGold and SoulSilver, the Armor Fossil is made available by using Rock Smash. Similarly to the method employed in Pokémon Diamond and Pearl, the player can take the fossil to the Pewter Museum to revive it into Shieldon. In Pokémon Black and White, the Armor Fossil is able to be received by talking to a miner in Twist Mountain, whom distributes a different fossil to passers-by daily. The Armor Fossil can be revived in the Unova region by taking it to the museum in Nacrene City. In X/Y, you are most likely to find it in Glittering Cave. You can then take it to the Fossil Lab in Ambrette Town.