ON THE SKULL OF TARSIPES ROSTRATUS. 3 
bones. Under the crescentic outgrowth of the squamosal, fits the crescent of the 
tympanic bone, with its concavity looking forwards, opposite to the other, and with 
a rounded convex rim posteriorly, behind which the seventh nerve emerges. This 
tympanic bone is not only firmly articulated with the surrounding bones, but is in 
part anchylosed both with the posterior part of the squamosal and with the hinder- 
most os bulle. This is a very rare condition in Marsupials. The inner and inferior 
portion of the bulla is formed by two shells of bone, which remain, so far, distinct 
from one another. The anterior (0.b.’) is continuous* in the neighbourhood of the 
Eustachian tube with the alisphenoid, and the posterior (0.b.”) already mentioned 
unites with the mastoid and partly with the true tympanic. 
The great bullate structure thus constituted differs from that of a Carnivore, 
for instance the Cat, by the absence of a septum, by the ossa bulle being apparently 
not preformed in cartilage, by these being double, and by the connection of the 
anterior with the alisphenoid. Owing to the great size of these ossa bulle, the 
cochlea is not indicated on the basal aspect of the skull as it is, for instance, in 
Kangaroos and Opossums. 
The basis cranii is long and becomes very slender in front. The presphenoid, 
a very slender rod, is distinct from the basisphenoid; the latter is longer but 
narrower than the basioccipital. From the ossified mesethmoid a long cartilaginous 
septum nasi runs forwards. 
The exoccipitals are large, the paroccipital processes small and short. The 
alisphenoids are very large and swollen, and lie entirely behind the orbitosphenoids, 
not overlapping them. The latter are very small bones, whose posterior margin is 
notched for the passage of the optic nerve. 
In the olfactory region, the turbinals are of simple form and ossified; long 
recurrent cartilages, forming the capsules of Jacobson’s organs, lie over the long 
palatine process of the premaxillaries. 
The mandible is as slender and delicate as in Echidna or Myrmecophaga. 
There is the very slightest possible coronoid elevation, and no angular process 
whatever ; the articular condyle is small, convex, and oval, with its height greater 
than its breadth. The dentary foramen is a long narrow groove, extending from 
the condyle along one-third the length of the bone. 
The malleus is rather large; its processes gracilis, once the continuation of 
Meckel’s cartilage, has become thin and leafy by absorption. The incus is normal ; 
the perforation in the stapes is small and triangular. The tensor tympani muscle 
is very large, and in its tendon I find a small oval nodule of bone (0.t.) which I 
have not detected in any other mammal; the stapedius is also com paratively a very 
large muscle. 
*The original distinctness of these elements has been traced by me in several species of Marsupials. 
It is a mistake to suppose that this anterior os bulla is only a backward process of the alisphenoid ; though 
such it appears to be in its even greater development in adult skulls of the Koala and the Hypsiprymni. 
