The Collaborative International Dictionary of English v.0.48:

Hypobranchial \Hy`po*bran"chi*al\, a. [Pref. hypo- + branchial.]
   (Anat.)
   Pertaining to the segment between the basibranchial and the
   ceratobranchial in a branchial arch. -- n. A hypobranchial
   bone or cartilage. Hypocarp