Upper jaw means the upper jaw extends past or equal to the lower. Lower jaw means the opposite, look at a few pics and you will understand.
Some examples of lower jaws: Senegal, ornate, delhizi, weeksii, tuegelsi, palmas (polli, buettikoferi, palmas), retropinnus
Upper jaws: ansorgii, endlicheri, congicus, lapradei, polypterus bichir bichir.
Most of the upper jaws, (besides ornate, tuegelsi, and weeksii) max out at 8-14", usually 10-12, so they will fit in a 55g.
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