The complete Pokemon RBY pokedex, part 9

Pokémon Name: Shellder
Type: Water
Classification: Bivalve Pokemon
Pokédex Number: 090
Ability: Shell Armor and Skill Link
Dream World ability: Overcoat
Useful Attacks: Aurora Beam
Location Found:
D/P/P: Route 205 South, Valley Windworks, Fuego Ironworks (Fish with Super Rod)
HG/SS: Routes 20, 21, 26, 27, 41, 47, New Bark Town, Pallet Town, Vermilion City, Cinnabar Island; Pokewalker - Blue Lake, Stormy Beach
B/W: Route 13, Undella Town, Undella Bay (all via Fishing, more common in bubble spots)

You'll want to keep Shellder in this state for as long as possible, as its evolution only learns a handful of moves on its own. A Water Stone is needed for the change, an item that is responsible for many Pokemon evolutions.

Evolution: To Cloyster via Water Stone