MARSUPIALIA. 



269 



Wombat the outwardly expanded arch is per- 

 fectly horizontal. In the Kangaroo the lower 

 margin of the zygoma describes a slightly undu- 

 lating curve, the middle part of which is con- 

 vex downwards. 



In many of the Marsupials, as the Kangaroo, 

 the Koala, some of the Phalangers, Petaurists, 

 and Opossums, the superior margin of the 

 zygoma begins immediately to rise above the 

 posterior origin of the arch. In the Wombat 

 an external ridge of bone commences at the 

 middle of the lower margin of the zygoma, and 

 gradually extends outwards as it advances for- 

 wards, and being joined by the upper margin 

 of the zygoma, forms the lower boundary of 

 the orbit, and ultimately curves downwards in 

 front of the ant-orbital foramen, below which 

 it bifurcates and is lost. This ridge results, as 

 it were, from the flattening of the anterior part 

 of the zygoma, which thus forms a smooth and 

 slightly concave horizontal platform for the eye 

 to rest upon. 



The same structure obtains, but in a slighter 

 degree, in the Koala. 



In the Kangaroo the anterior and inferior 

 part of the zygoma is extended downwards in 

 the form of a conical process, which reaches 

 below the level of the grinding-teeth. A much 

 shorter and more obtuse process is observable 

 in the corresponding situation in the Phalangers 

 and Opossums. 



The relative length of the facial part of the 

 skull anterior to the zygomatic arches varies re- 

 markably in the different Marsupial genera. In 

 the Wombat it is as six to nineteen ; in 

 the Koala as five to fourteen ; in the Pe- 

 taurus sciureus and Petaurus Bennettii it forms 

 about one-fourth of the entire skull ; in the 

 Phalangers about one-third ; in the carnivorous 

 Dasyures and Opossums more than one-third ; 

 in the Thylacine nearly one-half; in Perameles, 

 Macropus, and Hypsiprymnus murinus, III. the 

 length of the skull anterior to the orbit is equal 

 to the remaining posterior part ; but in a species 

 of Hypsiprymnus from Van Dieman's Land 

 (Hypsiprymnus myosurus, Ogilb.), the facial 

 part of the skull anterior to the orbit exceeds 

 that of the remainder, and the arboreal Hypsi- 

 pryinni from New Guinea present a still greater 

 length of muzzle. In most Marsupials the 

 skull gradually converges towards the anterior 

 extremity ; the convergence is more sudden 

 in the Petaurists, especially Pet. Bennettii; 

 but in the Peraineles lagotts the skull is re- 

 markable lor the sudden narrowing of the face 

 anterior to the orbits, and the prolongation of 

 the attenuated snout, preserving the same 

 diameter for upwards of an inch before it finally 

 tapers to the extremity of the nose. In the 

 Koala the corresponding part of the skull is 

 as remarkable for its shortness, as it is in the 

 Per. lagotis for its length, but it is bounded 

 laterally by parallel lines through its whole 

 extent. Before concluding this account of the 

 general form of the skull, I may observe that 

 in nearly all the Marsupials two long processes 

 project downwards from the inferior angles of 

 the occipital region ; they correspond in func- 



tion with, and have been described as the 

 mastoids, but they are developed from the 

 ex-occipital bones. These processes are longest 

 in the Kangaroos and Koala ; in the Wombat 

 they co-exist with the true mastoid pro- 

 cesses, which are of larger size. In the Opos- 

 sums and Dasyures the exoccipital processes 

 are short and obtuse ; in Acrobutcs they cease 

 to exist, but they are present in the larger 

 Petaurists. 



Of the composition of the cranium. The 

 occipital bone is developed, as in the placental 

 Mammalia, from four centres or elements, the 

 basilar below, the supra-occipital above, and 

 the ex-occipitals at the sides ; but these ele- 

 ments remain longer separate, and in some 

 genera do not become at any period of life 

 united by continuous 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. I 

 say not joined together, because in this Mar- 

 supial animal they meet above the foramen 

 occipitale and complete its boundaries, as the 

 corresponding superior vertebral laminae com- 

 plete the medullary canal in the region of the 

 spine. I find the same 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 the mature Wombat, of which 

 a reduced representation is given ztfig. 94, the 

 ex-occipitals were still unanchylosed ; the left 

 is figured separate at a. 



In the skull of a Perameles nasuta the ex- 

 occipitals are separated by an interspace, so 

 that a fissure is continued from the upper part 

 of the foramen magnum to the supra-occipital 

 element. The same structure may be observed 

 in the great Kangaroo, and it is very re- 

 markable in the young skulls of this species ; I 

 found this superior notch wide and well marked 

 in Macropus Bennettii. In the Wombat the 

 corresponding fissure is very wide, and the 

 lower margin of the supra-occipital is notched, 

 so that the shape of the foramen magnum 

 somewhat resembles that of the trefoil leaf. In 

 the Koala, the Phalanger, Petaurists, Ilypsi- 

 prymni, and Dasyurus Maugei, the elements of 

 the occipital bone present the usual state of bony 

 confluence. 



The temporal bone generally presents a per- 

 manent separation of the squamous, petrous, 

 and tympanic elements. I have observed this 

 reptilian-like condition of the bone in the ma- 

 ture skulls of an Ursine Dasyure, a Virginian 

 Opossum, a Perameles, in different species of 

 Potoroo and Kangaroo, in the Wombat, and in 

 the Koala. The petrous and mastoid elements 

 are commonly anchylosed together. So loose 

 indeed is the connexion of the tympanic bone, 

 that without due care it is very liable to be lost 

 in preparing the skulls of the Marsupials. In 

 the Kangaroo and Wombat (Jig. 94, b) it 



* Monographies de Mammalogie, pi. viii. 



