Bony tongues, in closer sense.
The order is characterised by internal anatomical features only. There are fifteen or fewer branched caudal fin-rays, except for some mormyrids. The nasal bone is gutter-like or irregularly rectangular (See Li and Wilson, 1996).
In body size they range from a few centimetres in the Stomatorhinus (Family Mormyridae) to about 4 meters in Arapaima (Family Osteoglossidae ) which is one of the largest living freshwater fishes and may reach 200 kg in weight.
Some taxa are air-breathers: Arapaima, Heterotis, Pantodon and Gymnarchus; mouth brooders are the species of Scleropages, and Osteoglossum ; nest-builders are Arapaima, Heterotis, and Gymnarchus,
This order consists of two suborders:
Suborder Osteoglossoidei
Suborder Notopteroidei