basioccipital bone
One of the bones of the occiput, the part of the skull which articulates with the spine. The basioccipital is located (if present) ventral to the foramen magnum. It often rests on or articulates with the basisphenoid, as well as meeting the exoccipitals. The basioccipital usually forms most (sometimes all) of the occipital condyle(s) and the ventral margin of the foramen magnum.