218 
MARINE SAUUIANS. 
serpents and fishes, where they act as barbs to 
prevent the escape of their prey.* 
The other parts of the skeleton follow the 
character indicated by the head. The vertebree 
are all concave in front, and convex behind ; 
being fitted to each other by a ball and socket 
joint, admitting easy and universal flexion. 
From the centre of the back to the extremity of 
the tail, they are destitute of articular apophyses, 
which are essential to support the back of 
animals that move on land : in this respect, they 
agree with the vertebra? of Dolphins, and were 
calculated to facilitate the power of swimming ; 
the vertebrae of the neck allowed to that part 
also more flexibility than in the Crocodiles. 
The tail was flattened on each side, but high 
and deep in the vertical direction, like the tail 
of a Crocodile ; forming a straight oar of im- 
mense strength to propel the body by horizontal 
* The teeth have no true roots and are not hollow, as in the 
Crocodiles, but when full ^own, are entirely solid, and united 
to the sockets by a broad and firm base of bone, formed from 
the ossification of the pulpy matter which had secreted the 
tooth, and still further attached to the jaw by the ossification 
of the capsule that had furnished the enamel. This indurated 
capsule, passed like a circular buttress around its base, tending 
to make the tooth an instrument of prodigious strength. The 
young tooth first appeared in a separate cell in the bone of the 
jaw, (PI. 20, h.) and moved irregularly across its substance, 
until it pressed against the base of the old tooth ; causing it 
gradually to become detached, together with its base by a kind 
of necrosis, and to fall off like the horns of a Deer. The 
teeth, in the roof of the mouth, are also constructed on the same 
principle with those in the jaw, and renewed in like manner. 
