The skull, in the Apoda, is remarkably solid and compact, and it possesses a postorbital or postfrontal bone (marked 1 in the figure) which does not exist in any of the other living batrachians.
In the typical group of the genus Phascolomys we find the following characters: - Fur rough and coarse; ears short and rounded; muzzle naked; postorbital process of the frontal bone obsolete; ribs fifteen pairs.
The wombat of Tasmania and the islands of Bass's Straits (P. ursinus), and the closely similar but larger P. platyrhinus of the southern portion of the mainland of Australia, belong to this group. On the other hand, in the hairy-nosed wombat (P. latifrons) of Southern Australia, the fur is smooth and silky; the ears are large and more pointed; the muzzle is hairy; the frontal region of the skull is broader than in the other section, with well-marked postorbital processes; and there are thirteen ribs.
- Tailed, lacertiform or serpentiform batrachians, with the temporal region of the skull roofed over by postorbital, squamosal, and supratemporal plates similar to the same bones in Crossopterygian fishes, and likewise with paired dermal bones (occipitals and post-temporals) behind the parietals and supratemporals.
Prefrontal bones are absent; post-frontals are possibly indicated by a frequently occurring separate centre of ossification in the postorbital process, to which the frontals always contribute.
Dogs, wolves, jackals, &c., which constitute the genus Canis in its more restricted sense, foxes are best distinguished by the circumstance that in the skull the (postorbital) projection immediately behind the socket for the eye has its upper surface concave, with a raised ridge in front, in place of regularly convex.
Sewellels are medium-sized terrestrial rodents, with no postorbital process to the skull, which is depressed in form, and rootless cheek-teeth, among which the premolars number I, the first in the upper jaw being very small.
In the skull the infra-orbital foramen is narrow, and postorbital processes and an alisphenoid canal are absent.