Halibut
(Pacific and California)
(Hippoglossus stenolepis and Paralichthys californicus)

Pacific Halibut are larger than California Halibut (full grown);
Pacific Halibut can get up to about 8 or 9 feet, while California get up to about 5 feet
(though personally I've think I've seen them bigger);
Pacific Halibut have concave ends of their tails, California are convex.
California Halibut are lefteye flounders; Pacific Halibut are righteye flounders.