Zoological Society. 44? 



of the brain, and for the great expansion of the nasal cavity imme- 

 diately 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 the 

 smaller herbivorous species, as the Petaurists and Potoroos, the era- 

 nium 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 the 

 length, and least in the Perameles lagotis in which it is less than 

 one-half. The occipital region, which is generally plane and ver- 

 tical 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 Kangaroo, and most so in the Opossum, 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 cha- 

 racter, which relates to the different development 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 for- 

 wards 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 ele- 

 vated sagittal crest, which, in mature skulls, is proportionally more 

 developed than in any of the placental Carnivora, not even exempt- 

 ing the strong-jawed Hyaena. 



Of the Composition of the Cranium. — *' The occipital bone is de- 

 veloped, as in the placental Mammalia, from four centres or ele- 

 ments, the basilar below, the supra- occipital above, and the ex-occi- 

 pitals at the sides ; but these elements remain longer separate, and 

 in some genera do not become, at any period of life, united by con- 

 tinuous ossification. 



" In the skull of an aged Virginian Opossum I found the supra- 

 occipital still distinct from the ex-occipitals, and these not joined 

 together, though anchylosed to the basilar element : in this Mar- 

 supial animal they meet above the foramen occipitale, and complete 



