Nelson asserts the monophyly of this group on the basis of their:

1) Symmetrical caudal fins, supported internally by one epural (bone).

2) Presence of first pleural rib on the second vertebrate (rather than third).

3) Presence of the pectoral girdle (low), with the postcleithrum bone scale-like.

et al.

... Many members of this Order are popular aquarium and experimental fishes.

Eight families, of about 88 genera, approximately 807 species per Nelson (1994).