Ichthyosaurus
Ichthyosaurus , noun
[New Latin, from Greek 'ichqy`s, -y`os, a fish + say^ros a lizard.]
(Paleontology) An extinct genus of marine reptiles; -- so named from their short, biconcave vertebra, resembling those of fishes. Several species, varying in length from ten to thirty feet, are known from the Liassic, Oolitic, and Cretaceous formations.