124 



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

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

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

 the forra of the tympanic bone in birds, but \ve have a still closef 

 resemhlance 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 ąuadruped the ty^mpanic, petrous, and mastoid 

 elements of the temporal bone are always anchylosed together ; this 

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

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

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

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

 that the enlargeraent of the tympanic cavity is due, in the Rodentia ; 

 and this expansion forms in that order, as is well kno\vn, a large 

 bulla ossea, \vhich is situated anterior and intemal to the mastoid 

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

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

 tbe pui-pose of iucreasing the extent of the auditory cavity ; but, with 

 one single exception, the Wombat, this bvlla 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 ala of the sphenoid bone. It is only 

 in the Perameles lagotis that, in addition to the preceding bulla. 

 I have observed an external dilatation of tha petrous element of 

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

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

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

 to the office of protecting the parts of the intemal 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 outs.de of the skull, viz. at the usual place at the base, where 

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

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

 toid portion appears betvveen the sąuamous, ex-occipital, and supra- 

 occipital bones. In the Wombat it sends outwards the strongcom- 

 pressed processwhich terminates the lateral boundaries of the occipital 

 plane of the cranium. 



" The auditory 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 ■vvith the tympanum is pro- 

 portionally greatest in the Petaurists, or Flying Opossuras, \vhere, 

 besides the sphenoid bulla, the mastoid element, and the -n-hole of 

 the zygomatic process of the temporal bone are expanded to form 

 air-cells \vith very thin and smooth waUs, 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 placentai IMammalia, with the habits of the species. 

 The meafus is directed outwards aud a little forwards in the ear- 



