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GENERAL OSTEOLOGICAL CHARACTERS 



OF 



EXISTING FORMS OF MARSUPIALIA. 



The foregoing sections have been applied to the restoration of extinct forms of Mar- 

 supial Mammals of which both species and genus have passed away. In now entering 

 upon that part of my work which deals with the determination of extinct species of 

 genera represented by living ones, it becomes requisite to premise the characters of the 

 petrifiable parts of such species, as grounds of the comparisons by which fossils of the 

 extinct species may be determined and made available for the work of reconstruction. 



Before entering upon such specific or generic osteological characters some observa- 

 tions on those of the Order may be acceptable. 



§ 1. The Skull. — The form of the skull offers much variety in the Marsupial order, 

 but is remarkable in both extinct and existing 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 one. The breadth of the skull, in relation to its 

 length, is greatest, after Nototherium, in the Wombat and Ursine Dasyure, in which it 

 equals three fourths of the length, and is least in the long-eared Bandicoot (Macrotis. 

 lagotis), in which it does not equal 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 a superoccipital 

 crest. This is least developed in Myrmecobius, Petaurus, and Macropus, and most so 

 in Thylacinus and the larger Opossums, in which, as also in the Koala, the crest curves 

 slightly backward, 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 develop- 

 ment of the temporal muscles, coordinate with the varieties of dentition in the different 

 genera. 



In the Wombats the coronal surface offers an almost flattened tract, bounded by two 

 slightly elevated and separate temporal ridges ; 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 upward from the line of their union a remarkably elevated sagittal 

 crest, which, in mature skulls, is proportionally more developed than in any of the 



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