CHAPTER XXIII.
GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA (continued).
THE SKULL AND APPENDICULAR SKELETON.
The Skull.
Monotremata. In both genera the cranium is thin-walled, has a fairly large cavity, and is very smooth and rounded externally. The sutures between many of the bones early become obliterated in a manner comparable to that in birds, and the facial portion of the skull is much prolonged.
In Echidna the face is drawn out into a gradually tapering rostrum, formed mainly by the premaxillae, maxillae and nasals. The zygomatic arch is very weak, and the palate extends very far back. The tympanic forms a slender ring. The mandible is extremely slight, with no ascending portion, and but slight traces of the coronoid process and angle. The hyoid has a wide basi-hyal and stout thyro-hyals, while the anterior cornua are slender, and include ossified epi-hyals and cerato-hyals.
In Ornithorhynchus the zygomatic arch is much stouter than in Echidna. The face is produced into a wide beak, mainly supported by the premaxillae, between whose diverging anterior ends there is a dumb-bell-shaped bone. The maxillae are flattened below, and each bears a large horny tooth, which meets a corresponding structure borne on a surface near the middle of the mandible. The mandible is considerably stouter than in Echidna, but the angle and coronoid process are but little developed. The infra-orbital foramen and the inferior dental and mental foramina of the mandible are all very large.
Fig. 90. Half front view[165] of the skulls of a Tasmanian Wolf