Ichthyosaurus 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.

Example:
What are the main Morphology in a Ichthyosaurus?

Taxonomy
Ichthyosaurus, ichthyosaur, fossils, Conybeare, free encyclopedia, Beiträge, communis, breviceps, fish, skeletons, baby, Maisch, MW, Stuttgarter, Naturkunde, Geologie und Paläontologie, 101, Lower Jurassic.

Defination from the web:
Ichthyosaurus (derived from Greek ιχθυς/ichthyos meaning 'fish' and σαυρος/sauros meaning 'lizard') is an extinct genus of ichthyosaur from the ... Some fossils still had baby specimens inside them, indicating that Ichthyosaurus was viviparous. ...