The Collaborative International Dictionary of English v.0.48:

Dibranchia \Dibranchia\ n.
   1. comprising all living cephalopods except the family
      Nautilidae: the orders Octopoda (octopuses) and Decapoda
      (squids and cuttlefish).

   Syn: Dibranchiata, subclass ta, Dibranchia, subclass
        Dibranchia.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Dibranchia
    n 1: comprising all living cephalopods except the family
         Nautilidae: the orders Octopoda (octopuses) and Decapoda
         (squids and cuttlefish) [syn: Dibranchiata, subclass
         Dibranchiata, Dibranchia, subclass Dibranchia]