plesiosaurus
noun
ple·si·o·sau·rus
ˌplēsēəˈsȯrəs
1
capitalized : a genus of marine reptiles (suborder Plesiosauria) of the Mesozoic of Europe and North America having a very long neck, a small head, and all four limbs developed as paddles for swimming
2
plural plesiosauri
-ȯˌrī
: any reptile of the genus Plesiosaurus 
Word History
Etymology
New Latin, from plesi- + -saurus