259 



the orbits ; the Biprotodon is more like the Kangaroos in the length of this part. The 

 terminal expanse and lateral tuberosities of the upper half of the bony nostril is a pecu- 

 liarity of Nototherium ; but it is instructive to note them in both Phascolarctos and 

 Phascolomys (Plate XXXVI. fig. 4) ; the fore part of the bony muzzle is expanded 

 laterally by an outward swelling of the front border of the premaxillary (ib. 22) where it 

 joins the nasal (ib. 15). 



In the form, especially breadth, of the external nostril the Notothere resembles the 

 Wombat, while the Diprotodon is more like the Kangaroo in this respect; but no 

 known existing Marsupial shows the septal plates developed from the premaxillaries at 

 the entry of the nasal passages, as in both Nototherium and Biprotodon. The Wombats 

 make the nearest approach to this peculiarity. 



The Notothere surpasses the Diprotodon in both the absolute and relative size of the 

 zygomatic arches. This difference is very striking when a front view of the cranium (as 

 in figure 2 of Plate XXXVI.) is compared with the similar view given of the Dipro- 

 todon's skull in Plate XIX. fig. 2. 



This most extraordinary feature in the cranial organization of the present large ex- 

 tinct Herbivore leads me to submit the following remarks. 



The zygomatic arches are relatively stronger and wider in Proboscidians than in 

 Ruminants and Solipeds ; they are widest and thickest in the bilophodont Dinotheres, 

 the temporal fossse being of corresponding capacity. Still more developed are these 

 arches in the Manatees, the Tapirs, and the bilophodont Megatheres, especially in 

 the vertical extension of the bone giving attachment to masticatory muscles. It would 

 seem that the working of opposed double-ridged grinders required greater strength 

 and more direct horizontal pull of the masseteric muscular fibres than the working 

 of the more complex but flatter molars of the Ox, Horse, Rhinoceros, or Elephant. 

 The phytophagous Marsupials have the grinding-surface of their many massive molars 

 raised into prismatic cones or transverse ridges, and their skull is remarkable for the 

 great strength, size, and span of the zygomatic arches. The descending process from 

 the fore and under part of the arch, for extending the origin of the premasseter muscle, 

 adds to the zygomatic complexities and characterizes the Po'ephaga among existing Mar- 

 supials. This osteological feature is not found in any gyrencephalous Herbivore ; but it 

 exists, with a different relation to the constituent bones of the arch, in the lissencephalous 

 Sloths, Megatherioids, and Glyptodonts. In the Xototheres the zygomatic development 

 reaches its maximum, with the dependent process extending from the maxillary element 

 of the arch as in other Marsupials. The muscular force operating on the mandible, both 

 for biting and chewing, was very great, indicative of unusual resistance in the alimentary 

 substances to be ground down. The grip of the front incisors brought by the shortness 

 of the face and jaw within the power of the crotaphyte muscles in a degree proportional 

 to the proximity of the inserted movers must have been like that of a vice. 



§ 3. The Mandible. A. Nototherium Mitchelli.— The mandible (Plate XXXVIII.) 

 discovered in the bed of King's Creek, a tributary of the river Condamine, Darling 



21* 



