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as in fish, united by squamous sutures ; and approximating 

 to fish in some parts of the structure of its vertebral column, 

 and in others, being sui generis. The four extremities are 

 decidedly sui generis^ and may be, in some respects, con- 

 sidered as forming a link between feet and fins : these are 

 called paddles. 



The following are the most essential particulars respect- 

 ing its structure : — Dentition. The teeth are not lodged in 

 separate alveoli, as in mammalia and crocodiles, nor do 

 they adhere by a bony union to the inner side of the maxilla, 

 as in other saurians, but lie loosely, retained only by the 

 fibrous substance of the gum in a continuous furrow formed 

 along the maxillae ; this furrow being only slightly contracted 

 between the teeth. The secondary teeth, which replace the 

 first set, are not formed in separate alveoli in the maxilla, but 

 close on the inner side of the roots of the first, which are 

 deeply excavated to receive them ; thus they penetrate 

 into and grow up within the interior of the old teeth, which 

 they finally push or rather lift out. All the appearances in 

 these stages of growth are absolutely conformable to those 

 exhibited in the teeth of the crocodile, excepting that, in the 

 crocodile, the old tooth, from the size of the conical alveolus 

 containing it, does not admit of being pushed out without 

 being previously split. But after these stages a difference 

 takes place, for in the crocodile, the teeth always remain 

 hollow, a very frequent and reiterated succession of new 

 teeth taking place; whereas, in the ichthyosaurus^ as in 

 monitors and other lacertce, their succession being less 

 frequent, the hollow becomes filled in age by the ossifica- 

 tion of the pulpy matter. This is the only point in which the 

 dentition of these animals agrees. The bones of the lower 

 jaw nearly resemble those of the crocodile, excepting that 

 the temporal muscle, rather than the masseter, appears to 

 have been employed in raising it, whereas in most species of 

 crocodile the former muscle is a mere rudiment : hence the 



Q q. 



