The facial portion of the skull is generally shorter than the cranial; the orbit is freely open behind; and the premaxillae tend to be reduced and fused with the nasals.
Nasal bones large and stout, co-ossified, and standing out freely above the premaxillae, from which they are separated by a deep and wide fissure; the latter small, generally not meeting in the middle line in front, often rudimentary.
Special interest attaches to the recent discovery in the cavern of Ultima Esperanza, South Patagonia, of remains of the genus Glossotherium, or Grypotherium, a near relative of Mylodon, but differing from it in having a bony arch connecting the nasal bones of the skull with the premaxillae; these include a considerable portion of the skin with the hair attached.
- The rodent skull is characterized by the great size of the premaxillae, which completely separate the nasals from the maxillae; by the presence of zygomatic arches; and by the wide unoccupied space existing between the incisors and the cheek-teeth; and (except in the Duplicidentata) by the antero-posteriorly elongated glenoid cavity for the articulation of the lower jaw.
The premaxillae are short and always enclose the nostrils.
- In the skull there is a sagittal crest; the tympanic bulla is filled with cancellous tissue; the condyle of the lower jaw is rounded; and the premaxillae, or anterior bones of the upper jaw, have the full number of incisor teeth in the young state, the outermost of these being persistent through life as an isolated tooth.
The skull is elongated, with an overhanging occiput, complete bony rims to the orbits, and the premaxillae separated from the arched and rather long nasals.
Gingival surfaces of the premaxillae and of Mn, Mandible.