Hard bony lump on inside gum line?

Torus Mandibularis ?: This may be the exostosis you are referring to. No treatment is usually necessary, but they may interfere with the fit of a mandibular denture. http://en.wikipedia.org/wiki/Torus_mandibularis

Mandibular torus: This is likely what you are noticing. Best to see your Dentist to be sure. It is a benign bony growth - usually bilateral- which typically stops growing by your 20's . It can be removed by the Oral Surgeon. Good luck.

Normal structure: are simply bony growths in the upper or lower jaws. These are normal anatomical structures. Tori are usually a clinical finding with no treatment necessary.