The Collaborative International Dictionary of English v.0.48:

Opisthomi \Op`is*tho"mi\, n. pl. [NL., fr. Gr. 'o`pisqen behind
   + ? the shoulder.] (Zool.)
   An order of eellike fishes having the scapular arch attached
   to the vertebrae, but not connected with the skull.
   [1913 Webster]