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,.
This buccal sac is provided with a dorsal mandible and a ventral radula.
With regard to the lower teeth the difficulties are greater, owing to the absence of any suture corresponding to that which defines the incisors above; but since the number of the teeth is the same, since the corresponding teeth are preceded by milk-teeth, and since in the large majority of cases it is the fourth tooth of the series which is modified in the same way as the canine (or fourth tooth) of the upper jaw, it is reasonable to adopt the same divisions as with the upper series, and to call the first three, which are implanted in the part of the mandible opposite to the premaxilla, the incisors, the next the canine, the next four the premolars, and the last three the molars.