and organ of hearing of the dugong. 147 
the upper table whatever, so that the process of absorption of 
the outer table of the skull, to make room for the formation 
of the permanent tusk, had not begun to take place. 
When the section of this tusk was compared with a similar 
section of the milk tusk of the narwhal, and of the elephant, 
I found that in its internal structure it closely resembled them ; 
and I noticed that the external surface in all of them was 
deficient in smoothness, when compared with the permanent 
tusks of those animals. 
In the narwhal, the root of the milk tusk terminates in a 
rounded blunt end, behind which there is no appearance of 
any preparation for the formation of the permanent tusk ; 
so that there is no evidence of that tusk having its origin in a 
cell similar to those of the dugong; but by comparing the 
place where the root of the milk tusk has its origin, with that 
of the permanent tusk in the same animal, for the two tusks 
in the narwhal come forward at such different periods of time, 
that this comparison may be made even when one permanent 
tusk appears to have arrived at its full growth, or nearly so ; 
at which time the rudiments of the corresponding permanent- 
tusk have not begun to be formed, and the depth in the skull 
of the milk and permanent tusk exactly correspond. 
As the permanent tusk in the narwhal begins to form in -a 
direct line immediately behind the origin of the milk tusk, 
the great purpose of the milk tusk is evidently to open the 
road for, and to direct the course of the permanent tusk, till 
it is completely pushed out by it. In the elephant, both of 
whose tusks appear at the same time, and whose skull en- 
creases in size much more rapidly than that of the narwhal, 
the permanent tusk has its origin even farther forward than 
