SKELETON OF MARSUPIALIA. 339 



are very small ; their coalesced bases arch backward over the 

 optic nerves and presphenoidal prolongation of the basisphenoid, 

 as in the bird, and their under part is grooved (not perforated) by 

 the optic nerves, which escape by the fissura lacera anterior. 



The nasal portion of the coalesced frontals is more expanded 

 than the cerebral one : the frontal sinuses extend to the coronal 

 suture and raise the outer far above the vitreous table : in this 

 table the frontal and coronal sutures remain, but they are obli- 

 terated in the outer table. The vomer is carinate below. The 

 nasals are distinct from each other and from the frontals : they 

 are grooved externally for the premaxillaries. The petromastoid, 

 tympanic, and temporal bones continue permanently separate, 

 though confluent ossification proceeds to blend the occipital, 

 parietal, and frontal into one bone. The petrosal is small, its 

 tentorial ridge or angle is sharp, and its cerebellar fossa very 

 deep, though small : a branch of the lateral sinus perforates the 

 petromastoid and the adjoining part of the temporal to open 

 behind the root of the zygoma : the mastoid part is compressed 

 and abuts against the outer side of the base of the paroccipital. 

 The tympanic is a simple scoop-shaped bone, or half-cylinder, cut 

 obliquely. The palatine process of the premaxillary is very 

 deeply notched, and is excavated behind the outer incisor. 



In the skull of the mature Wombat, fig. 220, the exoccipitals 

 were still unanchylosed ; the left is figured separate at 3. 



In the skull of a Perameles nasuta the exoccipitals are sepa- 

 rated by an interspace, so that a fissure is continued from the 

 upper part of the foramen magnum to the superoccipital element. 

 The same structure may be observed in the Great Kangaroo, and 

 it is very remarkable in the young skulls of this species. In the 

 Wombat the corresponding fissure is very wide, and the lower 

 margin of the superoccipital is notched, so that the shape of the 

 foramen magnum somewhat resembles that of the trefoil leaf. 

 In the Opossum, the exoccipitals meet above and complete 

 the foramen magnum. The petrosal and mastoid are commonly 

 confluent. So loose is the connection of the tympanic, that 

 without due care it is liable to be lost in preparing the skulls 

 of the Marsupials. In the Kangaroo and Wombat, it forms a 

 complete bony tube, fig. 220, 28, and in the Potoroo the bony 

 circle is incomplete at the upper part ; in the Perameles and 

 Dasyures the tympanic bone forms a semicircle, the posterior 

 part being deficient, and the tympanic membrane being there 

 attached to the mastoid, as in Birds. In the Dasyures, Petaurists, 

 Perameles, Potoroos, and Koala, fig. 221, c, there is a large bulla 



z 2 



