MOSASAURUS. 169 



whole skeleton, with such deviations only as tended to fit the 

 animal for its marine existence. 



The Mosasaurus had scarcely any character in common 

 with the Crocodile, but resembled the Iguanas, in having an 

 apparatus of teeth fixed on the pterygoid bone, (PI. 20. k.) 

 and placed in the roof of its mouth, as in many serpents and 

 fishes, where they act as barbs to prevent the escape of their 

 prey.* 



The other parts of the skeleton follow the character indi- 

 cated by the head. The vertebrge 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 sup- 

 port the back of animals that move on land: in this respect, 

 they agree M'ith the vertebrae of Dolphins, and were calcu- 

 lated to facilitate the power of swimming; the vertebras of 

 the neck allowed to that part also more flexibihty 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 immense strength to propel the body by 

 horizontal movements, analogous to those of skulling. Al- 

 though the number of caudal vertebrae was nearly the same 



* The teeth have no true roots and are not hollow, as in the Crocodiles, 

 but when full grown, 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 farther 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, (11. 20, h.) and moved irregularly 

 across its substance, until it pressed against the base of the old tooth ; causing 

 it gradually to beconie detached, together with its b■l^e by a kind of necrosist 

 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. 

 VOL. I. — 15 



