274 



MARSUPIALIA. 



ferous remains hitherto discovered in the se- 

 condary formations, will justify the minuteness, 

 perhaps tediousness, with which I have dwelt 

 on characters that, inclusive of the teeth, serve 

 to distinguish the cranium of the Marsupial 

 from that of any Placental quadruped. The 

 structure of the bony palate in the Marsupials 

 is interesting in other respects. Since the de- 

 fective condition of this part of the cranium is 

 one of the characteristics of the skull of the 

 Bird, it might be expected that some approx- 

 imation would be made to that structure in the 

 animals which form the transition between the 

 Placental and Oviparous Classes. We have 

 already noticed the large vacuities which occur 

 in the bony palate of nearly all the Marsupials ; 

 but this imperfectly ossified condition is most 

 remarkable in the great Perameles lagotis and 

 Acrobutes. In the former (fig. 96) the bony 



Fig. 96. 



Perameles lagotis. 



roof of the mouth is perforated by a wide 

 oval space extending from the second pre- 

 molars to the penultimate molars, exposing to 

 view the vomer and the convolutions of the 

 inferior spongy bones in the nasal cavity. Be- 

 hind this space there are six small perforations, 

 two in a transverse line midway between the 

 great vacancy arid the posterior margin of the 

 bony palate, and four in a transverse line close 

 to that margin. In Acrobates a still larger pro- 

 portion of the posterior part of the pulate is 

 formed by membrane. 



Cavity of the cranium. The parietes of the 

 cranial cavity are remarkable for their thickness 

 in some of the Marsupial genera. In the 

 Wombat the two tables of the parietal bones 

 are separated posteriorly for the extent of more 

 than half an inch, the insterspace being filled 

 with a coarse cellular diploe ; the frontal bones 

 are about two and a half lines thick. In the 

 Ursine Dasyure the cranial bones have a si- 

 milar texture and relative thickness. In the 

 Koala the texture of the cranial bones is denser, 

 and their thickness varies from two lines to half 

 a line. In the Kangaroo the thickness varies 

 considerably in different parts of the skull, but 

 the parietes are generally so thin as to be dia- 

 phanous, which is the case with the smaller 

 Marsupials, as the Potoroos and Petaurists. 

 The union of the body of the second with that 

 of the third cranial vertebra takes place in the 

 marsupial as in the-placental Mammalia at the 

 sella turcica, which is overarched by the back- 

 ward extension of the lesser alae of the sphenoid. 

 The optic foramina and the fissurae lacerae 

 anteriores are all blended together, so that a 

 wide opening leads outwards from each side of 

 the sella. Immediately posterior and external 

 to this opening are the foramina rotunda, from 

 each of which in the Kangaroo a remarkable 

 groove leads to the fossa Gasseriana at the com- 

 mencement of the foramen ovale ; the same 

 groove is indicated in a slight degree in the 

 Dasyures and Phalangers, but is almost ob- 

 solete in the Wombat and Koala. The carotid 

 canals pierce the body of the sphenoid, as in 

 Birds, and terminate in the skull very close 

 together behind the sella turcica, which is not 

 bounded by a posterior clinoid process. The 

 sphenoidal bulla, which forms the chief part of 

 the tympanic cavity in the Perameles lagotis, 

 forms a large convex protuberance on each side 

 of the floor of the cranial cavity in that species. 

 The petrous bone in the Kangaroo, Koala, and 

 Phalangers is impressed above the meatus 

 auditorius by a deep, smooth, round pit, which 

 lodges the lateral appendage of the cerebellum. 

 The corresponding pit is shallower in the Da- 

 syuri, and is almost obsolete in the Wombat. 

 The middle and posterior fissurae lacerae have 

 the usual relative position, but the latter are 

 small. The condyles are each perforated ante- 

 riorly by two foramina in most of the Mar- 

 supials, the Thylacinus forming the exception. 

 Of the composition and form of the foramen 

 magnum we have already spoken : it is of 

 great size in relation to the capacity of the 

 cranium ; the aspect of its plane is backward 

 and slightly downwards. 



In the Kangaroo and Phalan^er a thin ridge 

 of bone extends for the distance of one or two 

 lines into the periphery of the tentorial process 

 of the dura mater, and two sharp spines are 

 sent down into it from the upper part of the 

 cranium in the Plialangista vulpina. The ten- 

 torium is supported by a thick ridge of bone 

 in the Thylacine ; but it is not completely 

 ossified in any of the Marsupials : in some 

 species, indeed, as the Dasyures, the Koala, 

 and the Wombat, the bony crista above de- 

 scribed does not exist. There is no ossification 



