Longitudinal intrusion running over the inside and near the centre of the otolith. The sulcus can be divided in two separate parts; the anterior part is called the ostium and the posterior part is called cauda. Often the ostium and cauda are separated by a narrowing of the sulcus. When the ostium and cauda reach the margins of the otolith the sulcus is called open, if not the sulcus is closed. The sulcus can also be partly open, when only the ostium reaches the margin.