390 PROF. OWEN ON THE OSTEOLOGY 
The structure of the bony palate in the Marsupiata is interesting in other respects. 
Since the defective condition of this part of the cranium is one of the characteristics of 
the skull of the Bird, it might be expected that some approximation would be made to 
that structure in the animals which form the transition between the placental and ovipa- 
rous 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 re- 
markable in the Acrobates and Perameles lagotis'. In the latter the bony roof of the 
mouth is wanting for a wide oval space, extending from the second spurious molars to 
the penultimate molars, exposing to view the vomer and convolutions of the inferior 
spongy bones in the nasal cavity. Behind this space there are six small perforations, 
two in a transverse line, midway between the great vacancy and the posterior margin of 
the bony palate, and four in a transverse line, close to that margin. 
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 pari- 
etal bones are separated posteriorly for the extent of more than half an inch, the inter- 
space being filled with a coarse cellular diploé; the frontal bones are about two and a 
half lines thick. In the Ursine Dasyure the cranial bones have a similar 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 diaphanous ; which is the case with the smaller Marsupials, as the Potoroos and Pe- 
taurists. The union of the body of the second with that of the third cranial vertebra 
takes place in the Marsupiata, as in the placental Mammalia, at the sella turcica, which 
is overarched by the backward extension of the lesser ale of the sphenoid. The optic 
foramina and the fissure lacere anteriores are all blended together, so that a wide open- 
ing 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 remark- 
able groove leads to the fossa Gasseriana, at the commencement of the foramen ovale ; 
the same groove is indicated in a slighter degree in the Dasyures and Phalangers, but is 
almost obsolete in the Wombat and Koala. The carotid canals pierce the body of the 
sphenoid, as in the Bird, and terminate in the skull, very close together, behind the 
sella turcica, which is not bounded by a posterior clinoid process. The petro-tympanic 
bones form a large convex prominence on each side of the base of the cranial cavity in 
the Perameles lagotis ; this prominence is hollow, contains air, and has very thin walls. 
The petrous bone in the Kangaroo, Koala and Phalanger, 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 Dasyuri, and scarcely marked in 
the Wombat. The middle and posterior fissure lacere have the usual relative position, 
1 Pl. LXXI. fig. 1. 
