FISHES OF THE CHALK. 
133 
sharp and slightly curved ridges ; tliese are sur- 
rounded by a border of obtuse papillae. The 
Diodon Idstrix has one tooth of this kind affixed 
to the os hyoides, and another to the palate or 
roof of the mouth. But the fossil teetli are some- 
times found in considerable numbers, and of various 
sizes, forming a tesselated surface of several square 
inches ; and so regularly disposed, the smaller 
palates being adapted to the intervals between the 
larger ones, that no doubt can exist of this having 
been the mode in which they were placed in the 
original. Hence, instead of each specimen being 
a distinct palate, like the corresponding teeth of 
the Diodon, they appear to have constituted the 
covering of the entire roof and base of the mouth. 
PALATAL TOOTH OF AN EXTINCT SPECIES OF SHARK, OR 
DIODON. 
M. Agassiz (whose beautiful and truly scientific work 
on the Freshwater Fishes of Central Europe, affords 
an earnest of what may be expected from him when 
the history of the fossil fishes shall come under 
his examination) informed me, through Mr. Lyell, 
that he believed the teeth in question will be found 
K 3 
