The Collaborative International Dictionary of English v.0.48:

Postclavicle \Post*clav"i*cle\, n. [Pref. post- + clavicle.]
   (Anat.)
   A bone in the pectoral girdle of many fishes projecting
   backward from the clavicle. -- Post`*cla*vic"u*lar, a.
   [1913 Webster]