The basisphenoïd bone is an uneven bone, on the floor of the skull's cavity, rostraly to the basilar part of the occipital bone.
It is described by the french authors as the rear part of the sphenoïd bone (From the greak 'sphêno' ('corner') and 'eidés' ('in shape of')), whose front part is the presphenoïd bone.
These two pieces merge earlier in the human and later in most of the mammals.
Most of the times, they merge later with each other than with the basilar part of the occipital bone and especially with the ethmoïd bone.
Each of these two pieces has a body, median, and a pair of lateral expansions raised dorsaly, called wings.
Beside the wings, the basisphenoïd bone has on either side a strong process, the pterygoïd process.
The wings of the basisphnoïd bone are temporale; the ones of the presphenoïd bone are orbital.
They both participate in the constitution of the skull.