The ammonoid ventral rostrum is a relatively long projection which is located on the ventral side of the shell aperture. Most often the ventral rostrum is present on narrow streamlined ammonoid shells (oxycones, discocones) and shells with a promenent keel on the ventral side. In many taxa, the adult microconch shells have long rostra, whereas macroconchs and young ammonoids have short, rounded rostra. Functions of the ventral rostrum remain controversial. Most likely the rostrum could protect and maintain the ammonite hyponome during hydrojet movement (when the ammonite swam shell first).