380 PROF. OWEN ON THE OSTEOLOGY 
vertical plane surface behind, and in most of the species converging towards a point 
anteriorly ; it is also generally more depressed or flattened than in the placental Mam- 
mala. The skull is remarkable in all the genera for the small proportion which is 
devoted to the protection of the brain, and for the great expansion of the nasal cavity 
immediately anterior to the cranial cavity. 
In the stronger carnivorous species the exterior of the cranium is characterized by bony 
ridges and muscular impressions ; but in Myrmecobius and in the smaller herbivorous 
species, as the Petaurists and Potoroos, the craniwm presents a smooth rounded surface, 
as in Birds, corresponding with the smooth unconvoluted surface of the simple brain 
contained within. 
The breadth of the skull, in relation to its length, is greatest in the Wombat’ and 
Ursine Dasyure*, in which it equals three-fourths of the length, and least in Perameles 
lagotis*, in which it is less than one half. 
The occipital region, which is generally plane, and vertical in position, forms a right 
angle with the upper surface of the skull, from which it is separated by an occipital or 
lambdoidal crista. This is least developed in the Myrmecobius, Petaurists and Kan- 
garoo, and most so in the Thylacinus and larger Opossums, in which, as also in the Koala, 
the crest curves slightly backwards, and thus changes the occipital plane into a concavity, 
well adapted for the insertion of the strong muscles from the neck and back. The upper 
surface of the skull presents great diversity of character, which relates to the different de- 
velopment of the temporal muscles, and the varieties of dentition in the different genera. 
In the Wombat the coronal surface offers an almost flattened tract, bounded by two 
slightly elevated temporal ridges, which are upwards of an inch apart posteriorly, and 
slightly diverge, as they extend forwards to the anterior part of the orbit. 
The skull of the Opossum presents the greatest contrast to that condition, for the sides 
of the cranium meet above at an acute angle, and send upwards from the line of their 
union a remarkably elevated sagittal crest, which, in mature skulls, is proportionally 
more developed than in any of the placental Carnivora, not even excepting the strong- 
jawed Hyena. 
The Thylacine and Dasyures, especially the Ursine Dasyure, exhibit the sagittal crest 
in a somewhat less degree of development. It is again smaller, but yet well marked, 
in the Koala and Perameles. The temporal ridges meet at the lambdoidal suture in the 
Phalangers and Hypsiprymni, but the size of the muscles in these does not require the 
development of a bony crest. In the Kangaroo the temporal ridges, which are very 
slightly raised, are separated by an interspace of the third of an inch. They are sepa- 
rated for a proportionally greater extent in the Petaurists ; and in the smooth and con- 
vex upper surface of the skull of Pet. sciureus, Pet. pygmeus, and of Myrmecobius, the 
impressions of the feeble temporal muscles almost cease to be discernible. 
' Pl. LXXI. fig. 6. 2 Pl. LXX. fig. 5. 3 Pl. LXXI. fig. 1. 
5 § 
