170 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



opening remains, communicating with the cellular structure within, which bears 

 the strongest analogy to that occurring in many pachyderms. Moreover, the 

 several cranial bones exhibit a corresponding tendency to unite by anchylosis. 



" The brain-cavity is exceedingly small, as might be expected from the above 

 details ; and its long axis very nearly corresponds with that of the cranium, which 

 may be said to meet the general plane of the palate at an angle of 60". 



" The whole head may be divided longitudinally into three portions of nearly 

 equal breadth ; the brain-case, with its cellular parietes, &c., occupying the cen- 

 tral, and the zygomatic arches the lateral, parts, — the forehead, nose, and palate, 

 including the molar teeth, being equal in transverse measurement. The palatal 

 arch is single and smoothly rounded off, so that the posterior nasal spine is 

 absent. The lateral, thickened, and rounded nasal lobes are processes of the 

 premaxillary bone, which unite with the maxillaries in a curved vertical suture 

 with its concavity directed forwards. 



" The tail-like process projecting downwards and backwards from the anterior 

 and inferior part of the zygomatic arch has its homologue in the skull of many 

 animals, even in that of man himself. It is, therefore, jointly composed of the 

 malar process of the superior maxillary bone and the maxillary process of the 

 malar bone, united by a vertical suture. This projection must have given at- 

 tachment to the zygomatic muscles, and perhaps to a few of the anterior fibres 

 of the masseter, although extending below the level of the upper molars. That 

 of the left side has been slightly injured. 



" The zygomatic process of the temporal rests upon that of the malar bone, 

 with an oblique line of union running through the upper horizontal portion of 

 the arch. 



" The anterior convex border of the zygoma being nearly on a plane with the 

 external angular process of the os frontis, and the remarkable manner in which 

 the cranium is elevated on the facial bones, indicate very clearly that the eyes of 

 this animal were directed forwards, admitting of convergent vision. The low 

 position of the o'ptic foramen also shows that the eyes must have been situated 

 close to the orbital border (if it may be so called) of the superior maxilla." 



Notes on the Outline-drawings. — One of the peculiar features of 

 the skull of the phytophagous Marsupials, whether of the browsing 

 Kangaroos and Potoroos, or of the leaf-and fruit-eating Koalas, 

 Petaurists and Phalangers, or of the burrowing and root -gnawing 

 Wombats, is the great strength, size, and span of the zygomatic 

 arches, as compared with the answerable vegetable -eaters in the 

 Placental series of Mammalia*. 



This character is least marked in the true Kangaroos ; but it is 

 sufficiently distinctive of these as compared with the browsing or 

 grazing Pecora of similar size : and there is a modification of the 

 zygomatic arch in Macrojpus for extending the base of origin of the 

 masseter masticatory muscle, which is present in no gyrencephalousf 

 Herbivore, although it exist in the Hssencephalousf Sloths and 

 their great extinct herbivorous congeners the Megatherioids — I 

 allude to the descending process from the fore part of the zygomatic 

 arch (Plate YII. fig. 2, z). 



In all those herbivorous mammals in which the grinding teeth 

 present two transverse ridges, the zygomatic arches are well de- 

 veloped, the bony bar or plate being of great vertical extent : the 

 Tapir, the Manatee (Plate YIII. fig. 1, z, fig. 3, z), and the Megathere 



* See ' On the Osteology of the Marsupiaha,' Zool. Trans, ii. p. 387, pis. 69 

 and 71. 



t For the meaning of these terms see my ' Classification of the MammaHa,' in 

 the ' Proceedings of the Linnaean Society,' Feb. 17 and April 2l8t, 1857. 



