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.
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.
In the skull the infra-orbital foramen is narrow, and postorbital processes and an alisphenoid canal are absent.
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.