OF THE MARSUPIALIA. 383 
canal in the region of the spine. I have found the same structure and condition of the 
occipital bone of an adult Dasywrus Ursinus, and it is exhibited in the Plate of the era- 
nium of this species given by M. Temminck'. 
In the skull of Perameles nasuta the ex-occipitals are separated by an interspace, 
so that a fissure is continued from the upper part of the foramen magnum to the supra- 
occipital element. The same structure may be observed in the Kangaroo, and is very 
remarkable in the young skulls of this species. I found this superior notch wide, and 
well marked, in Macropus Bennettii. In the Wombat the corresportding fissure is very 
wide, and the lower margin of the supra-occipital is notched, so that the shape of the 
foramen magnum somewhat resembles that of the trefoil leaf. I found the ex-occipital 
element’ still unanchylosed with the basilar and supra-occipital elements in the cranium 
of a full-grown but not aged Wombat. 
In the Koala, the Phalangers, Petaurus, Hypsiprymnus, and Dasyurus Maugei, the ele- 
ments of the occipital bone present the usual state of bony confluence. 
The temporal bone generally presents a permanent separation of the squamous, 
petrous and tympanic elements. I have observed this reptile-like condition of the bone 
in the mature skulls of an Ursine Dasyure, a Virginian Opossum, a Perameles, in diffe- 
rent species of Potoroo and Kangaroo, in the Wombat, and in the Koala. So loose, in- 
deed, is the connexion of the tympanic bone, that, without due care, it is very liable 
to be lost in preparing the skulls of the Marsupiata. In the Kangaroo and Wombat? it 
forms a complete bony tube, about half an inch in length, with an irregular exterior, and 
is wedged in between the mastoid and articular processes of the temporal bone. In the 
Potoroo the bony circle is incomplete at the upper part ; in the Perameles and Dasyures 
the tympanic bone forms a semicircle, the posterior part being deficient, and the tym- 
panic membrane being there attached to a descending process of the squamous element 
of the temporal. Here we have 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 Rep- 
tiles, in its want of union with 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 pro- 
cess behind the petro-tympanic portion. It is to the expansion of the petro-tympanic, 
and not of the mastoid, portion of the temporal bone, that the enlargement of the tym- 
panic cavity is due, in the Rodentia ; 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 Marsupiata, as the Dasyures, Petaurists, Perameles, Potoroos, 
and Koala, there is also a large bulla ossea for the purpose of increasing the extent of the 
auditory cavity ; but, with one single exception—the Wombat, this bulla is not formed 
' Monographies de Mammalogie, Pl. VIII. * Pl. LXXI. fig. 6 a. 3 Pl, LXXI. fig. 6 6. 
VOL. II.—PART y. 3E 
