on the Fossil Oencra Ichthyosaurus and Plesiosaurus. 107 



2dly, The appearance and progress of the new tooth, which is to replace 

 the old one, is very nearly the same in the ichthyosaurus and in the croco- 

 dile. This will be best seen by comparing- figs. 4, 5, and 6, PI. XV. which 

 represent the dentition of the former, with figs. 1 and 2, which represent that of 

 the latter. In fig. 4. is seen a tooth of an ichthyosaurus, in which a fissure has 

 been effected in the side of the root, by the growth of the new tooth. In fig. 2, 

 a crocodile's tooth under the same circumstances, an exact agreement will be 

 observed in this respect. Fig. 5. represents a section passing longitudinally 

 along the jaw, and exhibiting the new tooth_, which has penetrated the inte- 

 rior cavity of the old one. Fig, 5, b, shows the appearance of the upper sur- 

 face of this specimen, where the tops of the old and new tooth are broken 

 off. Fig. 6. represents a similar section passing transversely across the jaw- 

 bone. In this figure the jaw-bone itself and the canal which traverses it for 

 the passage of the nerves and vessels are exhibited. These two figures may 

 be compared with fig. 1, which represents the appearance of the new tooth 

 of the crocodile, when it has penetrated the interior cavity of the old one. 

 The agreement will be found exact. But if a comparison be instituted with 

 fig. 3, (representing the dentition of the fossil animal of Maestricht, and of 

 the lacertae distinguished by generic characters from the crocodile,) a great 

 difference will be seen : for in these last two cases the new tooth, instead of 

 shooting into the interior of the old one, shoots irregularly across the osseous 

 mass, incoi'porating it with the jaw-bone. 



It should be observed, that in all these instances the new tooth, when it first 

 makes its appearance, consists only of a hollow conical shell of enamel, which 

 increases in length by the ap^wsition of osseous matter at the lower edge, 

 gradually forming its root. 



3dly, In pursuing, however, the history of the teeth of the ichthyosaurus to 

 the last stage, we quit these analogies with the crocodile, and arrive at another 

 point, wherein the ichthyosaurus resembles the other laccrtas, in common with 

 many of the mammalia. This is the gradual obliteration of the interior cavity 

 in old age, by the ossification of the pulpy nucleus. Fig. 13. PI. XV. repre- 

 sents a transverse section of the root of a tooth, in which this process has 

 taken place. The ossified pulp has become a spongy mass of reticulated bony 

 fibres ; and the osseous laminae forming the exterior of the root, are seen sur- 

 rounding it in wavy cortical layers ; not, however, continuing all round the 

 tooth, but interrupted by openings corresponding to the furrows that mark 

 the outside of the tooth ; between which openings the spongy substance insi- 

 nuates itself. The only inference from this filling up of the interior cavity in 



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