344 ANATO^NIY OF VERTEBRATES. 



different genera ; they are longest and narrowest in the Perameles, 

 shortest and broadest in the Koala, fig. 221, i^ Their most cha- 

 racteristic strncture is the expansion of their upper and posterior 

 extremity, which is well marked in the Wombat, Myrmecobius, 

 Petaurists, Phalangers, Opossums, and Dasyures. In the Po- 

 toroos the anterior extremities of the nasal bones converge to 

 a point which projects beyond the premaxillaries. In the Perameles 

 lagotis the bony case of the nasal passage is further increased 

 by the presence of two small rostral bones, resulting, as in the 

 Hog, from ossification of the nasal cartilage. 



The premaxillaries always contain teeth, and the ratio of the 

 developement of these bones corresponds mth the bulk of the 

 dental apparatus which they support. They are consequently 

 largest in the Wombat, where they extend far upon the side of 

 the face and are articulated to a considerable proportion of the 

 nasals, but do not, as in Rodentia, reach the frontal or divide the 

 maxillary bone from the nasal. They present a somewhat lower 

 degree of developement in the Koala, fig. 221, 22, but both in 

 this species and in the Wombat they bulge outward, and thus 

 remarkably increase the transverse diameter of the osseous cavity 

 of the nose. In Hypsiprymnus and Macropus the incisive palatal 

 foramina are chiefly in the premaxillaries, but a small proportion 

 of their bony circumference is due to the anterior extremity of 

 the palatal process of the maxillary : the same structure obtains 

 in the Wombat, Koala, and Opossums. In the Dasyures and 

 Phalangers a greater proportion of the posterior boundary of the 

 incisive foramina is formed by the maxillaries ; in the Petaurists 

 they are entirely surrounded by the maxillary bones, while in the 

 Perameles they are, on the contrary, entirely included in the 

 maxillaries. They always present the form of two longitudinal 

 fissures, fig. 222, a. 



The maxillary, fig. 221, 21, in the Koala and Wombat sends 

 upward a long, narrow, irregular nasal process, which joins the 

 frontal and nasal bones, separating them from the premaxillaries. 

 The antorbital foramen does not present any marked variety of 

 size, which is generally moderate. It is much closer to the orbit in 

 the carnivorous Marsupialia than in the corresponding placental 

 quadrupeds. It is relatively largest in the Ursine Dasyure. It 

 presents the form of a vertical oblique fissure in the Wombat. I 

 have observed it double in the Kangaroo. In this and some 

 other herbivorous Marsupials the malar process of the maxillary 

 sends down a process for increasing the poAver and size of the 

 masseter muscle. 



