supernumerary (extra) rib

Results from the retention & development of the costal processes of cervical & lumbar vertebrae. The distal part of the transverse processes of these vertebrae develops from the costal processes, which in some people grow unduly large forming extra rib.

References

Clinically Oriented Anatomy, 4th edition, KL Moore & AF Dalley (eds), Lippincott Williams & Wilkins, Philadelphia, PA 1999.