SKELETON OF MAESUPIALIA. 341 



outward and a little forward in the carnivorous Dasyures ; out- 

 ^v ard and a little backward in the Perameles and Phalangers ; 

 outward, backward, and upward in the Kangaroos, and directly 

 outward in the Petaurists and Wombat ; but the differences of 

 direction are but slightly marked in these timid vegetarians. 



The squamosal generally reaches half-way from the root of the 

 zygoma to the sagittal ridge or suture ; it is most developed in 

 the Wombat, in which its superior margin describes a remarkably 

 straight line. The zygomatic process is generally compressed 

 and much extended in the vertical direction in the Opossum, 

 Dasyure, Phalanger, Koala, fig. 221, 27, and Kangaroo. In the 

 Wombat it curves outward from the side of the head in the form 

 of a compressed and almost horizontal plate, fig. 220, 27 ; it is 

 then suddenly twisted into the vertical position, to be received 

 into the notch of the malar portion of the arch, ib. 26. In Macro- 

 pus the back part of the zygoma is perforated by a pneumatic 

 foramen which receives air from the tympanum. 



The cavity corresponding to the sphenoidal bulla ossea in other 

 Marsupials is in this species excavated in the lower part of the 

 squamosal at the inner side of the articular surface for the lower 

 jaw. This articular surface, situated at the base of the zygomatic 

 process, presents in the marsupial as in the placental Mammalia 

 various forms, each manifesting a physiological relation to the 

 structure of the teeth and adapted to the required movement of 

 the jaws in the various genera. In the herbivorous Kangaroo 

 the glenoid ca\dty forms a broad and slightly convex surface, as 

 in the Ruminants, aflPording freedom of rotation to the lower jaw 

 in every direction. In the Phalangers and Potoroos the articular 

 surface is quite plane. In the Perameles it is slightly convex 

 from side to side, and concave from behind forward. In the 

 Wombat it is formed by a narrow convex ridge considerably ex- 

 tended, and slightly concave, in the transverse direction. This 

 ridge is not bounded by any descending process posteriorly, so 

 that the jaw is left free for the movements of protraction and re- 

 traction. In the Koala the glenoid cavity is a transversely oblong 

 depression with a slight convex rising at the bottom, indicating 

 rotatory movements of the jaw. In the carnivorous Dasyures 

 it forms a concavity still more elongated transversely, less deep 

 than in the placental Carnivora, but adapted, as in them, to a 

 ginglymoid motion of the lower jaw. In all the genera, save in 

 the Wombat, retraction of the lower jaw is opposed by a de- 

 scending process of the temporal bone immediately anterior to the 

 meatus auditorius and tympanic bone. 



