The Collaborative International Dictionary of English v.0.48:

Abdominal \Ab*dom"i*nal\, n.; E. pl. Abdominals, L. pl.
   Abdominales.
   A fish of the group Abdominales.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Abdominales \Ab*dom`i*na"les\, n. pl. [NL., masc. pl.] (Zool.)
   A group including the greater part of fresh-water fishes, and
   many marine ones, having the ventral fins under the abdomen
   behind the pectorals.
   [1913 Webster]