Familiarity breeds accuracy
The best way to differentiate between the Upper and Lower Jaw Bichirs is to memorize them.
This is by no means an easy task because of the number of Polypterus species in existence. The lone E. calabaricus is a standalone species, so it is easy to take to memory as an Upper Jaw Bichir. But the Bichirs from the Polypterus genus are harder to recall because there are 15 described species and a host of undescribed ones.
However, I have devised a simple system for this. Ever noticed that as a kid it was quite easy to count in multiples of five? Whether playing hide and seek or reciting in Math class, counting by 5s was easy: 5, 10, 15, 20, 25…
To classify which among the described Bichirs under the genus Polypterus belong to the Upper or Lower Jaw variety, simply rely on the first three multiples 5, 10, and 15. There are 5 Lower Jaw Bichirs, 10 Upper Jaw Bichirs and 15 Described Bichirs.
Once again, don’t forget the E. calabaricus under the Erpetoichthys genus, which is an Upper Jaw Bichir.