The mandible is composed of several bones as in reptiles.
The operculum of the normal zooecium has become the mandible, while the occlusor muscles have become enormous.
They include the mandible of a mastodon and a portion of a vertebra of a large fish, both found in the Lower Madison Valley; the skull and other parts of a dog (Mesocyon drummondanus), found near Drummond, Granite county; the skull of a Poatrephes paludicola, found near New Chicago,.
The isolated gnathobase often seen as " mandible " and the genital operculum).
The upper surface of the lateral edges of the mandible has also a number of parallel fine transverse ridge, like those on the bill of a duck.