124 



a semicircle, the posterior part being deficient, and the tympanic 

 membrane being there attached to a descending process of the squa- 

 mous element of the temporal. Here we iiave a near approach to 

 the form of the 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 quadruped the tympanic, petrous, and mastoid 

 elements of the temporal bone are always anchylosed together ; this 

 condition is well shown in the skull of the Porcupine and Beaver, 

 in which the mastoid element sends down a thick obtuse process be- 

 hind the petro-tympanic portion. It is to the expansion of the pe- 

 tro-tympanic and not of the mastoid portion of the temporal bone 

 that the enlargement of the tympanic cavity is due, in the Hodentia ; 

 and this expansion forms in that order, as is well known, a large 

 bulla ossea, which is situated anterior and internal to the mastoid 

 process. In many of the Marsupials, as the Dasyures, Pctaurists, 

 Perameles, Potoroos, and Koala, there is also a large bulla ossea for 

 the pui'pose of increasing the extent of the auditory cavity ; but, with 

 one single exception, the Wombat, this bulla is not formed by the 

 tympanic or any other element of the temporal bone, but by the ex- 

 pansion of the base of the great a/a of the sphenoid bone. It is only 

 in the Perameles lagotls that, in addition to the preceding bulla, 

 I have observed an external dilatation of the petrous element of 

 the temporal bone, which thus forms a second and smaller bulla on 

 each side, beliind the large bulla ossea formed by the sphenoid. In 

 other Marsupiata the petrous bone is of small size, generally limited 

 to the olhce of protecting the parts of the internal ear, and some- 

 times, as in the Koala, is barely visible at the exterior of the base of 

 the skull. The petrous and mastoid elements are commonly anchy- 

 losed together. In the Kangaroos, Koala, and Wombat, the petro- 

 mastoid bone is of a large size, and is visible in two situations 

 on the outside of the skull, viz. at the usual place at the base, where 

 the petrous portion is wedged in between the basilar bone, ex-occipital 

 and sphenoid ; and again at the side of the cranimn, where the mas- 

 toid portion appears between the squamous, ex-occipital, and supra- 

 occipital bones. In the Wombat it sends outwards the strong com- 

 pressed process which terminates the lateral boundaries of the occipital 

 plane of the cranium. 



" The auditor}^ chamber of the ear is augmented in the Phalangers, 

 the Koala, the Kangaroo, and Potoroo, by a continuation of air-cells 

 into the base or origin of the zygomatic process; but the extent of 

 the bony air-chambers communicating with the tympanum is pro- 

 portionally greatest in the Petaurists, or Flying Opossums, where, 

 besides the sphenoid bulla, the mastoid element, and the whole of 

 the zygomatic process of the temporal bone are expanded to form 

 air-cells with very thin and smooth walls, thus presenting an inter- 

 esting analogy in the structure of the cranium to the class of birds. 



" The direction of the bony canal of the organ of hearing corre- 

 sponds, as in the jolacental Mammalia, with the habits of the species. 

 The meatus is directed outwards and a little forwards in the car- 



