This group has at times been classified with the sharks : the similarities are obvious , not only do the chondrosteans mostly lack bone , the structure of the jaw is more akin to that of sharks than other bony fish . Both lack scales ( except the Polypteriforms ) . This group has at times been classified with the sharks : the similarities are obvious , not only do the chondrosteans mostly lack bone , the structure of the jaw is more akin to that of sharks than other bony fish , and both lack scales ( excluding the Polypteriforms ) .