Type III is a fusion of a large secondary ossification center resulting in a large medial portion of the tuberosity of the navicular, referred to by some as "cornuate navicular." (5,6,9,12,13,16)
The type III accessory navicular, or cornuate navicular, is a prominent navicular tuberosity, which is essentially a type II ossicle connected to the medial aspect of the parent navicular by an osseous bridge.