HELIX. 



249 



second set often, and sometimes a third set of teeth are 

 seen through the transparent base of the shell, irregularly 

 striated, but generally having equal spaces between each 

 two sets. They are apparent in the youngest as well as 

 in the oldest specimens, and continue to be formed from 

 time to time, so long as the shell increases in size. They 

 probably mark regular periods of growth ; and it may be 

 that these are annual. The growth seems to go on 

 actively for a time, by the addition of new testaceous mat- 

 ter, indicated by the oblique stride, and then alternates 

 with a season of repose, when the teeth and aperture are 

 formed. 



The teeth appear never to be entirely absorbed and 

 removed, although the aperture, near which they were 

 originally placed, is often advanced very far beyond them. 

 When in motion, the shell lies horizontally on the animal's 

 back. 



A curious subject of investigation is the albinism, or 

 entire absence of coloring matter, in the shells of certain 

 individuals of this and other species. The albinos of 

 this species are of a pure, lively white, while the con- 

 tained animal is highly colored. Mr. Anthony remarks 

 that about one-seventh of all the specimens collected by 

 him, in the neighborhood of Cincinnati, are colorless. As 

 they are apparently operated upon by the same phy- 

 sical agents which influence the others, it is not easy to 

 conjecture how this singular cfiect is produced. The 

 animal is sometimes cream-colored throughout ; but in 

 such instances the shell is usually colored. 



VOL. ii. 32 



