THE OllNITHORYNCHUS PAllABOXUS. S5 
able resemblance to the incus of the Mammalia, and has 
precisely the same situation ; lastly. The vertical direction 
in which it is placed, shows that it cannot be a process of 
the malleus. In thus reducing the ossicula of the ear to an 
analogy with those of the Mammalia, I am not unwilling 
to admit, that there are certain points of resemblance with 
those of reptiles and birds, more particularly in the semi- 
cartilaginous state of the bones themselves, in the exposed 
situation of the cavity of the tympanum, in the very pecu- 
liar form of the stapes ; but it must be evident, I think, to 
all, that certain of these deviations from the usual structure 
arise out of the deficiency in the osseous parietes of the 
tympanic cavity, and consequent attachment of the small 
bones of the ear, and the membrana tympani itself, to a 
portion of the pterygoid processes. On the other hand, the 
internal aspect of the cavity of the tympanum appears quite 
regular ; the vestibular foramen, that which in Man, by a 
faulty nomenclature, is called oval, is in the Ornithorynchus 
perfectly circular. An attempt is made, by a projection of 
the boneS;, to divide the tympanum into two cavities, in one 
of which is situated the ossicula auditus, and the vestibular 
opening. By the wide opening into the other cavity passes 
the facial nerve, which escapes from the cavity by a hole 
close to the cochlear opening of the internal ear ; this latter 
is much larger than the vestibular. The principal branch 
arising from the superior cervical ganglion passes along the 
roof of the tympanum, and internally with regard to the 
stapes, on its way to join the fifth pair of cerebral nerves. 
It will be readily understood that the cavity of the tym- 
panum, which is inclosed partly by osseous, and partly by 
cartilaginous parietes, is considerable. Its anterior portion 
may be divided into two principal cavities, viz. that which 
contains the ossicula and vestibular foramen, and that 
