Occasional Papers of the Museum of Zoology 15. 



normal form, although sometimes evident breaks give excep- 

 tions to this. 



Pilsbry (1895), in his classic on the Helicidse, has pointed 

 out that modification of the radula tends to take place from the 

 center out. How^ever, in some groups of snails the radula 

 tends to be concave, as is also the odontophore. This is espe- 

 cially noticeable in the Helicinidac and Neritidse, where there 

 are two radular cartilages with a slit between. In these it 

 would seem that the greatest stress would tend to come at two 

 points some distance out from the center on either side. 



Coincident with this stress-tendency, the greatest adaptive 

 modification in some groups seems to be, not at the very center 

 of the radula, but at some distance out on either side. Thus, 

 in the Zonitidie the inner marginals appear to show the greatest 

 modification along certain lines, while in the Helicinidse the 

 lateral complexes are certainly the most highly specialized teeth. 

 In those radula of this type which show progressive modifica- 

 tion within the transverse row, as, for example, in Euconulus 

 and Ciuppya ( Habroconus), the adaptive (?) specialization 

 seems to become less toward the inside as well as toward the 

 outside (compare Part III of present paper). 



In Oxv^tyla prince ps the radula is concave at the center 

 and wear appears to be greatest some distance out from the 

 center on each side. On account of the high degree of modi- 

 fication of all of the teeth, it is difiicult to establish the posi- 

 tion of greatest adaptive (?) modification, but it perhaps may 

 be assumed to be near the position of greatest stress. For 

 this reason, I am inclined to regard these lanceolate teeth as 

 vestigial or atavistic rather than as nascent modifications. 



The variability in the numbers of these peculiar teeth and 

 their occurrence in some individuals in most of the genera of 



