172 
MARINE SAURIANS. 
diles, but are ranged in one long continuous 
furrow, (PI. 11, B, c), of the maxillary bone, in 
which the rudiments of a separation into distinct 
alveoli may he traced in slight ridges ex- 
tending between the teeth, along the sides and 
bottom of the furrow. The contrivance by 
which the new tooth replaces the old one, is 
very nearly the same in the Ichthyosauri as in 
the Crocodiles (PI. 11 , a, b, c) ; in both, the 
young tooth begins its growth at the base of 
the old tooth, where, by pressure on one side, 
it causes first a partial absorption of the base, 
and finally a total removal of the body of the 
older tooth, which it is destined to replace.* 
As the predaceous habits of the Ichthyosauri 
exposed them, like modern Crocodiles, to fre- 
quent loss of their teeth, an abundant provision 
has in each case been made for their continual 
renewal. 
* In PI. 11, Fig. A, shews the manner in which the older 
tooth in the Crocodile becomes absorbed, by pressure of a younger 
tooth rising within the cavity of its hollow base. Fig. c, repre- 
sents a transverse section of the left side of the lower jaw of an 
Ichthyosaurus, shewing two teeth in their natural place, within 
the furrow of the jaw; the younger tooth, by lateral pressure, 
has caused absorption of the inside portion of the base of the 
older tooth. Fig. b, represents a transverse section of the entire 
snout of an Ichthyosaurus, in which the lower jaw e,xhibits on 
both sides, a small tooth (a), which has caused partial absorption 
of the base of the larger tooth, (c). In the upper jaw, the 
bases of two large teeth (d, d,) are seen in their respective 
furrows. 
