MARSUPIALIA. 



271 



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 of the temporal 

 bone is generally compressed and much ex- 

 tended in the vertical direction in the Opossum, 

 Dasyure, Phalanger, Koala, and Kangaroo. 

 In the Wombat it curves outwards from the 

 side of the head in the form of a compressed 

 and almost horizontal plate; it is then sud- 

 denly twisted into the vertical position, to be 

 received into the notch of the malar portion of 

 the arch. 



The cavity corresponding to the sphenoidal 

 bulla ossea in other Marsupials is in this 

 species excavated in the lower part of the 

 squamous element of the temporal bone 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 cavity forms a broad and slightly 

 convex surface, as in the Ruminants, affording 

 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 forwards. In 

 the Wombat it is formed by a narrow convex 

 ridge considerably extended, and slightly con- 

 cave, in the transverse direction. This ridge is 

 not bounded by any descending process pos- 

 teriorly, so that the jaw is left free for the 

 movements of protraction and retraction. But 

 this structure is widely different from that 

 which facilitates similar movements in the Ro- 

 dentia. In these there is a longitudinal groove 

 on each side, in which the condyle of the lower 

 jaw plays backwards and forwards, but is im- 

 peded in its lateral movements ; these, on the 

 contrary, are freely allowed to the Wombat, 

 and the oblique disposition of the lines of 

 enamel upon the molar teeth correspond with 

 the various movements of which the lower jaw 

 of the Wombat is thus susceptible. In the 

 Koala the glenoid cavity is a transversely ob- 

 long 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, biit 

 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 descending process of the temporal bone 

 immediately anterior to the meatus auditorius 

 and tympanic bone. 



The glenoid cavity presents a characteristic 

 structure in most of the Marsupialia in not 

 being exclusively formed by the temporal bone. 

 With the exception of the Petaurists, the malar 

 bone forms the outer part of the articular sur- 

 face for the lower jaw, and in the Tliylacinus, 

 Dasyurus Mangel, Dasyurus ursinus, Pera- 



meles, Hypsiprymnus, and Macropus the sphe- 

 noid ala forms the inner boundary of the same 

 surface ; but this ala does not extend so far out- 

 wards and backwards in the Wombat or Koala. 



The sphenoid bone has the same general 

 form and relative position as in the ordinary 

 Mammalia, but in many Marsupials it presents 

 a similarity to that in the Ovipara in the per- 

 sistence of the pterygoid processes as separate 

 bones, as shown in the Wombat (fig. 94, c). 

 It is only in the Koala that I have observed 

 a complete obliteration of the suture joining 

 the basilar element of the sphenoid with 

 that of the occipital bone. In the Thylacine 

 a narrow straight bridge of bone is continued 

 from the auditory sphenoidal bulla forwards to 

 the base of the pterygoid process, resembling 

 the condition of the pterygoids in Birds. 



The chief peculiarity in the sphenoid bone 

 is the dilatation of the root of the great ala 

 already alluded to. This dilatation communi- 

 cates and is filled with air from the tympanum. 

 It forms the hemispherical bulla ossea on each 

 side of the basis cranii in the Dasyures and 

 Phascogales, and the large semi-ovate bullie in 

 the Myrmecobius, Cook's Phalanger, &c. ; but 

 in the Koala the bulls (b, Jig. 95,) are still 



Fig. 95. 



Phascolarctus. 



more developed, and are produced downwards 

 to an extent equal with the ex-occipital pro- 

 cesses (a, fg. 95) ; they are somewhat com- 

 pressed laterally, and instead of the smooth 

 and polished surface which characterizes them 

 in the preceding genera, terminate here in a 

 rough ride;e. The dilated air-chambers or 

 bullse of the sphenoid are very small in the 

 Thylacine ; in the Phalangers and Potoroos 

 they are relatively smaller than in the Dasyures, 

 and they are incomplete posteriorly in the Kan- 

 garoos and Wombat. In the Brush-Kangaroo 

 the above process from the sphenoid joins the 

 base of the large descending process of the 

 ex-occipital. The pterygoid processes are 

 relatively largest in the Kangaroo, Wombat, 

 and Koala, and present in each of these species 

 distinct hamular processes. In the Potoroo, 

 Kangaroo, and Wombat the sphenoid ala com- 

 bines with the pterygoid process to form a 

 large and deep depression opening externally. 

 In the Kangaroo, Dasyures, Koala, and Wom- 

 bat the great alae of the sphenoid articulate 

 with the parietal bones, but by a very small 

 portion in the two latter species : in the Pera- 



