152 Sir Everard Home on the milk tusks, 
bottom, allows itself to sink by its own weight, descending 
in a standing posture, so that as soon as the head is covered, 
the water would pass into the tube of the ears, were there 
not something like a valve at the orifice of the meatus ex- 
ternus to prevent it. In other animals that live in the sea, 
or are much under water, there are contrivances, very dif- 
ferent from this, to prevent the water getting into the tube 
of the ear. In the whale tribe, the external orifice is so 
extremely small, as to exclude it. In the seal, the meatus 
externus makes a turn nearly circular, to answer the same 
purpose. In the ornithorhyncus paradoxus, the external 
opening is at a great distance from the organ ; and the 
meatus, which is the size of a crow quill, and cartilaginous, 
winds round upon the temporal bone. The external meatus 
in the walrus, I have not had an opportunity of examining ; 
the orifice in the bony tube corresponds in size with that of 
the hippopotamus, its termination at the membrana tympani 
is less oblique, and its direction horizontal. The ossicula in 
the hippopotamus are small, the stapes is imperforate, and 
the bones have no bony union. 
The external orifice of the meatus externus in the dugong 
is extremely small, so as readily to exclude the water. The 
cochlea is very small, making only one turn and nearly one 
half. The semicircular canals are also exceedingly small. 
The peculiar bony connections to the tympanum, connecting 
the malleus and incus with the bones of the skull already 
described, lead to the idea that this animal is more indebted 
for its hearing, than any other that lives in water, to the 
vibrations received by the bones of the skull, being commu- 
