There were sup­posed to be only two primary lines of descent, the eukaryotic and the prokary­otic.

The anterior rami terminate at about the mid-level of the orbit, and the medial rami articulate firmly with the anterolateral corner of the otic capsule.

Laterally this bone is deeply emarginate; posteriorly it forms a large part of the otic notch, through which the columella passes.