398 PROCEEDINGS OF THE ACADEMY OF [1898. 



5. In certain species and groups there are forms of teeth in the 

 embryonal, or larval stage, entirely different from those found in 

 post- embryonal life, so that we may properly speak of the change as 

 a metamorphosis. 



6. The new formation and transformation of teeth is, as a rule, 

 not exactly symmetrical on the two sides of the radula, so that a 

 formula of one side is often only approximately true. 



7. The width of the individual teeth, (and of the longitudinal 

 rows), is the same from the earliest formation to the later stages and 

 the increase in width of the whole radula is effected exclusively by 

 the addition of new longitudinal rows. 



<S. The central teeth seem to be" (and in some species are doubt- 

 less) simple from their first formation ; not a product of the coales- 

 cence of two original laterals. 



9. They are, as a rule, more or less markedly asymmetrical. 



10. The terms "lateral" and "marginal" teeth are not of ab- 

 solute significance: The "marginals" of a younger specimen, or a 

 part of them, will have transformed into laterals when the animal is 

 adult. 



11. It is also quite inadmissible to speak of the marginals as 

 modified laterals, for a transformation never takes place in this 

 direction. We should not say, as has been usual, that the simple 

 mesodont of the laterals becomes split, or bicuspid, in the marginals; 

 but the contrary is true : the "double pointed mesodont" of the 

 marginal loses its endodont and becomes simple as the tooth is grad- 

 ually replaced by a lateral. It is incorrect to say: "The teeth 

 become smaller towards the margins," as they virtually increase in 

 size from the margin toward the middle. 



12. The expression "transition teeth" between marginals and 

 laterals (not the reverse) has become of a real, actual meaning, just 

 as the word "relationship" among organisms has gained its proper 

 meaning by the theory of descent, 



13. The different ways in which the teeth are formed and trans- 

 formed will probably furnish valuable hints for systematic mala- 

 cology. 



''At least in the land Pulmonata (Nephropneusta v. lb.): in some Limnaei- 

 dae. e. g. Limiiua palustris Mull., there is a strong probability that the cen- 

 trals are double in first formation. 



