270 



MARSUPIALIA. 



Fig. 94. 



Phascolomys. 



forms a complete bony tube, about half an 

 inch in length, with an irregular exterior ; 

 it is wedged in between the mastoid and 

 articular processes of the temporal bone. In 

 the Potoroo the bony circle is incomplete at 

 the upper part; in the Perameles and Da- 

 syures the tympanic bone forms a semicircle, 

 the posterior part being deficient, and the tym- 

 panic membrane being there attached to a des- 

 cending process of the squamous element of 

 the temporal bone. Here we have a near ap- 

 proach to the form of tympanic bone in Birds, 

 but we have a still closer resemblance to its 

 condition both in Birds and Reptiles, in its 

 want of union with and relations to the petrous 

 element of the temporal bone. In the Rodent 

 quadrupeds the tympanic, petrous, and mas- 

 toid elements of the temporal bone are always 

 nnchylosed together; this condition is well 

 shown in the skull of the Porcupine and 

 Beaver, in which the mastoid element sends 

 down a thin obtuse process behind the petro- 

 tympanic portion. It is to the expansion of 

 the petro-tympanic, and not of the mastoid 

 portion of the temporal bone, that the enlarge- 

 ment, of the tympanic cavity is due in the 

 Rodentia, and this expansion forms in that 



order, as is well known, a hirge bulla OSKCO, 

 which is situated anterior and internal to the 

 mastoid process. In many of the Marsupials, 

 as the Dasynres, Petaurists, Perameles, Po- 

 toroos, and Koala, there is also a large bulla 

 os^ca for the purpose of increasing the extent of 

 the auditory cavity ; but, with one single ex- 

 ception, the Wombat, this bulla is not formed 

 by the tympanic or any other element of the 

 temporal bone, but by the expansion of the 

 base of the great ala of the sphenoid bone. In 

 Acrobates and Perameles lagotis, in addition 

 to the preceding bulla there is also an external 

 dilatation of the petrous element of the tem- 

 poral bone, which thus forms a second and 

 smaller bulla on each side, behind the large 

 bu!la ossea formed by the sphenoid. In other 

 Marsupials the petrousbone is of smallsize, gene- 

 rally limited to the office of protecting the parts 

 of the internal ear, and sometimes, as in the 

 Koala, is barely visible at. the exterior of the base 

 of the skull. The petrous and mastoid elements 

 are usually anchylosed together in the Marsu- 

 pials, and the mastoid portion appears in the 

 occipital region of the skull of the Koala, 

 between the ex-occipital bones and squamous 

 portion of the temporal. The petrous element 

 of the temporal bone appears externally in 

 the corresponding part of the skull of a young 

 Emeu. In the Kangaroos and Wombat the 

 petro-mastoid bone presents a larger size, and 

 is visible in two situations on the outside of 

 the skull, viz. at the usual place at the basis, 

 where the petrous portion is wedged in be- 

 tween the basilar bone, ex-occipital and sphe- 

 noid, and again at the side of the cranium, 

 where the mastoid portion appears between the 

 squamous, ex-occipital, and supra-occipital 

 bones. In the Wombat it sends outwards the 

 strong compressed process which terminates 

 the lateral boundaries of the occipital plane of 

 the cranium ; but this process is entirely due 

 to the ex-occipitals in the Koala and other 

 Marsupials. 



The auditory chamber of the ear is aug- 

 mented in the Phalangers, the Koala, the Kan- 

 garoos, and Potoroo, by a continuation of air- 

 cells into the base or origin of the zygomatic 

 process ; but the extent of the bony air-cham- 

 bers communicating with the tympanum is 

 proportionally greatest in the Flying Opossums, 

 where, besides the sphenoid bulla, the mastoid 

 element and the whole of the zygomatic pro- 

 cess of the temporal bone are expanded to 

 form air-cells with very thin and smooth walls, 

 thus presenting an interesting analogy in the 

 structure of the cranium to the class of birds. 



The direction of the bony canal of the organ 

 of hearing corresponds, as in the placental 

 Mammalia, with the habits of the species. 

 The meatus is directed outwards and a little 

 forwards in the carnivorous Dasyures ; out- 

 wards and a little backwards in the Perameles 

 and Plialangers ; outwards, backwards, and 

 upright in the Kangaroos, and directly out- 

 wards in the Petaurists and Wombat; but the 

 differences of direction are but slightly marked. 



The squamous element of the temporal bone 



