382 PROF. OWEN ON THE OSTEOLOGY 
flattening of the anterior part of the zygoma, which thus forms a smooth and slightly 
concave horizontal platform for the eyeball to rest upon. 
The same structure obtains, but in a slighter degree, in the Koala. In the Kangaroo 
the anterior and inferior part of the zygoma is extended downwards, in the form of an 
inverted conical process, which reaches below the level of the grinding teeth. A much 
shorter and more obtuse process is observable in the corresponding situation in the 
Phalangers and Opossum. ‘The relative length of the facial part of the skull, anterior to 
the zygomatic arches, varies remarkably in the different marsupial genera. In the 
Wombat it is as 6 to 19; in the Koala as 5 to 14; in the Phalangers it forms about one- 
third the length of the entire skull; in the carnivorous Dasyures and Opossums it is 
more than one-third. In Perameles, Macropus, and Hypsiprymnus murinus, Ill., the 
length of the skull anterior to the orbit is equal to the remaining posterior part ; but in 
a species of Hypsiprymnus from Van Diemen’s Land (Hypsiprymnus myosurus, Ogilb.), 
the facial part of the skull anterior to the orbit exceeds that of the remainder ; and the 
arboreal Hypsiprymni from New Guinea present a still greater length of muzzle. In 
most Marsupiata the skull gradually converges towards the anterior extremity ; but in 
the Perameles lagotis' the skull is remarkable for the sudden narrowing of the face anterior 
to the orbits, and the prolongation of the attenuated snout, preserving the same diameter 
for upwards of an inch before it finally tapers to the extremity of the nose. In the 
Koala the corresponding part of the skull is as remarkable for its shortness as it is in 
the Perameles lagotis for its length, but it is bounded laterally by parallel lines through 
its whole extent. Before concluding this account of the general form of the skull, I 
may observe that the Kangaroo resembles the placental Ruminantia, and some Rodentia, 
as the Viscaccia, in the prolongation downwards of two long processes, corresponding 
in function with the mastoid, but developed from the ex-occipital bones. The same pro- 
cesses are developed in an almost equal degree in the Koala’, and, in the Wombat, co- 
exist with a corresponding development of the true mastoids. The ex-occipitals each 
send down a short obtuse process in the Potoroos, Perameles, Petaurists, Phalangers, 
Opossums, and Dasyures. 
Of the Composition of the Cranium.—The occipital bone is developed, as in the pla- 
cental Mammalia, from four centres, or elements,—the basilar below, the supra- occipital 
above, and the ex-occipitals at the sides ; but these elements remain longer separate, 
and in some genera do not become, at any period of life, united by continuous ossifi- 
cation. 
Tn the skull of an aged Virginian Opossum I found the supra-occipital still distinct 
from the ex-occipitals, and these not joined together, though anchylosed to the basilar 
element. In this marsupial animal they meet above the foramen occipitale, and complete 
its boundaries, as the corresponding superior vertebral Jamine complete the medullary 
1 Pl. LXXI. fig. 1. 2 Pl. LXIX. figg. 1. & 3. a. 
