accessory rib

https://doi.org/10.1351/goldbook.10323
Rib arising from a cervical vertebra (cervical rib), or supernumerary rib arising from a thoracic or lumbar vertebra.
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 715 [Terms] [Paper]