Splenial

The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side (closest to the tongue) between the angular and surangular.^[1][2]

Hemimandible of Trimerorhachis; the splenial (labeled "sp") is visible on the interior surface

Skull and jaws diagram of the primitive synapsid Dimetrodon, showing location of the splenial