What's the difference between ichthyosaur and ichthyosaurus?

Ichthyosaur

Definition:

• (n.) One of the Ichthyosaura.

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Ichthyosaurus

Definition:

• (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.

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