of Abdominal
A group including the greater part of fresh-water fishes, and many marine ones, having the ventral fins under the abdomen behind the pectorals.
Source: Webster's dictionaryFishes lacking this appendage were placed within the order Apodes; fishes containing abdominal, thoracic, or jugular pelvic fins were termed Abdominales, Thoracici, and Jugulares respectively. Source: Internet