ichthyosaurus

n. (also ichthyosaur) (pl. -sauruses or -saurs) extinct marine reptile with four flippers and usu. A large tail. [greek ikhthus fish, sauros lizard]

n. An extinct genus of marine reptiles; -- so named from their short, biconcave vertebrae, resembling those of fishes. Several species, varying in length from ten to thirty feet, are known from the Liassic, Oolitic, and Cretaceous formations.