SKELETON OF MARSUPIALIA. 3:^5 



anteriorly : it is also generally more depressed or flattened than 

 in the placental Mammalia. The skull is also remarkable in all 

 the Marsupial genera for the small proportion which is devoted 

 to the protection of the brain, and for the great expansion of the 

 nasal ca\dty immediately anterior to the cranial cavity. 



In the stronger carnivorous Marsupials the exterior of the cra- 

 nium is characterised by bony ridges and muscular impressions, 

 but in the smaller herbivorous and insectivorous species, as the 

 Petaurists, Potoroos, and Myrmecohius, the cranium presents a 

 smooth convex surface as in Birds, corresponding with the smooth 

 unconvoluted surface of the simple brain contained within, fig. 

 219. 



The breadth of the skull in relation to its length is greatest in 

 the Wombat,^ Ursine Dasyure,^ and Petaurists, in which it equals 

 three-fourths of the length, and is least in the 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 crista is least developed in the Myrmecobius, Petaurists, 

 and Kan2;aroos, and most so in the 



. /-x • 219 



Thylacine and larger Opossums, m 

 which, as also in the Koala, the 

 crest curves slightly backward, and 

 thus changes the occipital plane 

 into a concavity for the firm im- 

 plantation of the strong muscles p,,.... ,.,,«.»., .magnified. 

 from the neck and back. 



The upper surface of the skull presents great diversity of 

 character, which relates to the different developement of the 

 temporal muscles, and the varieties of dentition in the different 

 genera. 



The extinct Nototherium offers the singular exception of an 

 expansion of the facial part of the skull, vertically and trans- 

 versely, from the orbits to the terminal muzzle.^ 



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 forward to the anterior part of the orbit. In the 

 skull of the Virginian Opossum the sides of the cranium meet 

 above at an acute angle, and send upward from the line of their 



> As 15 to 20. 2 As 10 to U. 



^ Lxxxiir. p. 170, pi. vii. 



