204 Mr. Carlisle on the Physiology of the Stapes. 
remarkably to depress its subjacent end into the fenestra 
vestibuli ; besides the thickness of the basis, and its exact 
adaptation to the fenestra, exhibit a joint of considerable 
motion. In those animals there is only a small perforation, 
instead of the crural arch. Vide letter n* 
I have discovered a very remarkable singularity, in tracing 
the comparison of this bone, in the marmot, and Guinea-pig. 
The stapes in these animals is formed with slender crura, con- 
stituting a rounded arch, through which an osseus bolt passes, 
so as to rivet it to its situation. This bolt I have named pes- 
sulus. Vide letter /. It is placed near the top of the arch, so 
that by the action of the stapedeus muscle the upper part of 
the straight crus is brought into contact with the pessulus; 
and by this means the depression of the basis is limited. It 
does not seem obvious for what further end this provision is 
designed, because, excepting the shrill whistle, there is nothing 
peculiarly different in the habits of those animals from others 
which are destitute of such mechanism . 
The kanguroo has this bone like the corresponding ossicle 
in birds, called Columella ; but it has also the malleus and 
incus, which birds have not. 
In the ornithorhynchus paradoxus, and ornithorhynchus hys- 
trix, the resemblance to the columella is still more striking ; 
and forms an additional point of similarity between these 
strange quadrupeds and birds. Their columella? are not, how- 
* The stapes of the seal has solid rounded crura, and a small aperture; that of the 
walrus is entirely solid, and the edges as well as the plane of the sides, are a little 
twisted, agreeing with the observation of M. Cuvier, Lemons (P Anaiomie compare'e , 
Tome II. p. 505. In all these aquatic mammalia the fenestra rotunda, called also 
fenestra cochlea, is large, being three or four diameters more than in other animals 
of similar buikc 
