Hypostome (trilobite)

This article is about the trilobite structure. For other organisms, see Hypostome.
A partially complete, forked, conterminant hypostome of the trilobite Isotelus (Ordovician of southern Ohio).

The hypostome is the hard mouthpart of trilobites found on the ventral side of the cephalon (head). The hypostome can be classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella.