Genjintch is a bit similair in gameplay to Mothra. In Mothra for the game you have to guess which one of the four holes the moths are in and with Genjintch you'll have to guess where the bone is hidden.
When Genjintch gets sick you'll have to give him plants to heal him. The prehistoric man eats meat and mushrooms, since they had no donuts at that time ;-). He likes to make some pottery and you can praise him when he has created something, this will increase his evolution level.
I've scanned some images from the original package