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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Origin and Etymology of tetrabranchia
New Latin, from tetra- + -branchia
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