DEVELOPMENT OF TEETH IN HATTERIA PUNCTATA. 197 



be restricted to the replacement of Nos. 1 and 2 by a succes- 

 sional tooth, and that even this may not always occur. 



Passing on to the teeth of the lower jaw, we find that the 

 anterior region, which in the adult is occupied by a large 

 tooth on either side, has in the newly-hatched animal three 

 separate teeth in the corresponding positions. We have seen, 

 also, that the posterior two of these are replaced by a succes- 

 sional tooth, and that there are signs of the deciduous nature 

 of the first. My material does not take me far enough to 

 decide whether any of the teeth of the last young individual 

 described take part in the formation of the adult tooth. The 

 presence of a well-developed residual dental lamina in this 

 region at this stage proves that further change may occur. 

 Concerning the second successioual, it is quite possible that 

 it may be a component of the adult compound tooth, Avhich 

 I believe is formed partly at the expense of the anterior teeth 

 of the alternating series, their vertical successors taking part 

 in its constitution. In any case I believe the constituent 

 teeth to be three in number on each side, though in the adult 

 this is rarely evident.^ Newman (22) appears to have had a 

 specimen iu which the middle cusp was especially distinct. 

 The ''fusion" is produced in the same manner as in the case 

 of the large incisors. In the adult jaw the large anterior 

 tooth is succeeded by a short ridge, which is the representa- 

 tive of the posterior portion of the alternating- series. The 

 teeth are practically worn down to the bone, which is there- 

 fore, as in some parts of tlie upper jaw, the functional equi- 

 valent of the teeth. From the attachment of the large 

 anterior teeth to the jaw there is a marked step to the level 

 of this ridge. I have observed that in the falling out of 

 deciduous teeth iu the lower jaw, the amount of bone that is 

 separated along with the tooth by absorption is very great. 

 The alternating series of the lower jaw very soon cease 

 growth, and are hence worn away at an early stage, the 

 bone here growing very rapidly and becoming dense and 

 solid. This cessation of growth is no doubt due to their 

 ' Two. See Addendum. 



