123 



through its whole extent. Before concluding this account of the 

 general form of the skull, I raay observe that the Kangaroo resemblea 

 the placentai Ruminantia and some Rodentia, as the Viscaccia, in the 

 prolongation dowiiwards of t\vo long processes corresponding in 

 function to the mastoid, but developed from the exoccipital bones. 

 The šame processes are developed in £in almost equal degree in the 

 Koala, and, in the Wonibat, coexist with a corresponding develop- 

 ment of the true mastoids. The exoccipitals each send do\vn a short 

 obtuse process in the Potoroos, Perameles, Petaurists, Phalangers, 

 Opossums, and Dasyures. 



0/the Composition of the Cranium. — "The occipital bone is de- 

 veloped, as in the placentai 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 

 its boundaries, as the corresponding superior vertebral lamince com- 

 plete the medullary canal, in the region of the spine. I have found 

 the šame structure and condition of the occipital bone of an adult 

 Dasyurus Ursinus, and it is exhibited in the plate of the cranium of 

 this species given by M. Temminck*. In the skull of a Perameles 

 nasuta the ex-occipitals were separated by an interspace, so that a 

 fissure was continued from the upper part of the foramen magnum to 

 the supra-occipital element. The šame structure may be observed 

 in the Kangaroo, and is very remarkable in the young skuUs of this 

 species ; I found this superior notch wide and well-marked in Ma- 

 cropus Bennettii. In the Wombat the corresponding fissure is very 

 wide, and the lower margiu of the supra-occipital is notched, so 

 that the shape of the foramen magnum somewhat resembles that of 

 the trefoU leaf. In the Koala, the Phalanger, Petaurus, Hypsiprym- 

 nus, and Dasyurus Maugei, the elements of the occipital bone present 

 the usual statė of bony confluence. 



" The temporal bone generally presents a permanent separation 

 of the sąuamous, petrous, and tympanic elements. I have observed 

 this reptile-like condition of the bone in the mature škulis of an 

 Ursine Dasyure, a Virginian Opossum, a Perameles, in difFerent spe- 

 cies of Potoroo and Kangaroo, in the "VVombat, and in the Koala. 

 So loose, indeed, is the connection of the tympanic bone, that, 

 ■without due care, it is very liable to be lošt in preparing the skuUs 

 of the Marsupiata. In the Kangaroo and ^^^ombat it forms a com- 

 plete bony tube, about half an inch in length, with an irregular ex- 

 terior, and is \vedged in between the mastoid and articular processes of 

 the temporal bone. In the Potoroo the bony circle is incomplete at the 

 upper part; in the Perameles and Dasyures the tympanic bone forms 



• Monographie de Mammalogie, pi. viii. 



