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equally simple; their growth is by layers, as in mammifera? ; 

 but it never extends to the formation of roots fixed into 

 alveoli. As in monitors, and some other saurians, they con- 

 sist merely of the crown, and when this part is complete, the 

 pulpy nucleus upon which they are formed becomes ossified. 

 When the tooth must be shed, it breaks off; the ossified foot 

 becomes united with, and is converted into part of the jaw. 

 There is, however, some exception to this rule ; as, for exam- 

 ple, in the anarrhicas, whose bony nuclei being prominent, 

 and larger than the teeth themselves, are shed with them in a 

 manner probably analogous to the shedding of deer's antlers. 

 Teeth are reproduced during a great part of the life of fishes, 

 and to all appearance without fixed periods ; the new tooth 

 rising before, beside, or behind, the one it is destined to re- 

 place. Vertical reproduction is most common for round teeth, 

 which rise from a cavity seated deeper than the nucleus of the 

 preceding, and pierce the bone after the former is obliterated. 

 Those which are replaced laterally are mostly the great 

 conical hooked, or cutting teeth ; they pierce the bone by the 

 side of the tooth still in its place. 



Among the more curious dentitions are those of the inferior 

 pharyngeals of carps, forming half collars in the pharynx ; 

 and the triangular enamelled plate in the basilary bone. 

 Those of scari, forming a kind of parrot's bill. Those of 

 tetrodons and diodons, whose entire jaw is armed with only 

 two, or even one compound tooth. Those of chimaerse and 

 mylobates, also compound teeth, rising from a multitude of 

 germs in the form of filaments, and forming plates covered with 

 a common enamel. Those of lampreys, consisting of thin 

 cornets, formed upon fleshy moulds. Finally, in squali, the 

 nucleus of the teeth remains for a long time cartilaginous and 

 flexible, so that the reserve teeth are laid back against the 

 palate ; but the moment a necessity comes for their employ- 



