260 HELICID^. 



tions have led to doubt. Specimens of the young of 

 Helix interna resemble it so exactly, on the lower surface, 

 that it is impossible to distinguish them except by count- 

 ing the internal teeth, which in Helix inte)-na, never, so 

 far as I have noticed, exceed three in each row. The 

 aperture of both is also the same, and the color not very 

 different. The superior sui-face, however, shows consid- 

 erable differences. The spire in this is flatter, the whorls 

 are more numerous by at least one full volution, and it is 

 smooth and shining, and entirely destitute of prominent 

 ribs. It never attains one-fourth part of the bulk of that 

 shell. Yet, notwithstanding these differences, the gen- 

 eral aspect of the young of that species, and what are 

 considered to be the mature of this, are so nearly the 

 same, that it is difficult to refrain from considering them 

 to be identical. If we ever know the modifications which 

 are produced in the external characters of this genus, by 

 the influence of external causes, it will very probably be 

 found that this is only a variety of the preceding species. 

 And that there will be, gradually, a fusion of received 

 species into those which are typical and, as it were, cen- 

 tral species, there can be no doubt. 







