plural noun
Tet·ra·bran·chia | \-aiŋ-\

Definition of Tetrabranchia 

: a subclass or order of Cephalopoda including among existing forms only the genus Nautilus, differing from the remaining existing cephalopods in having four gills, four auricles to the heart, and a large chambered external shell, and including many extinct forms — see ammonoidea , nautiloidea — compare dibranchia

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History and Etymology for Tetrabranchia

New Latin, from tetra- + -branchia

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