26 THE NAUTILUS. 



hatching, all sizes may be found in the fall, none of which will be 

 full grown. The members of this species require more than one 

 year in which to mature, while other species deposit the ova early in 

 May and the young attain full size the same season. Having a 

 knowledge of which species mature respectively in one, two and 

 three seasons of growth greatly simplifies the determination of 

 species. 



It is unnecessary to watch the growing snail, as I have done, to 

 learn this fact, for every shell carries unmistakable evidence of the 

 number of periods of growth it has required to attain its present size. 

 This evidence is the white and brown lines upon the last, and next 

 to the last whorl, called variceal lines, or bands. At the end of the 

 season, or when the pond dries up, or when from any other cause the 

 shell ceases to grow, the snail strengthens the shell by constructing 

 a triangular-shaped rib within the sub-margin of the lip. In forming 

 this rib the coloring matter which exudes from the mantle of the 

 animal is withheld until the structure is complete, thus leaving a 

 white or light-colored line on the outside of the shell immediately 

 over the rib. Sometimes the line will be shaded with brown or 

 some dark color on one side, rarely on both sides. As the ribs are 

 never removed, the lines always remain visible on the outside of the 

 shell, and mark the periods of growth. 



For brevity in describing this feature I may be permitted to coin 

 the word annuan. It is derived from the Latin word annus, year, and 

 the suffix an, pertaining to, and signifies pertaining to one year, or 

 period of growth. 



In examiuining shells, first determine whether the example is an 

 annuan, bi-annuan or tri-annuan ; then examine the general form 

 and appearance of the shell, the texture, sculpture, the form of the 

 sutures, showing how the whorls are joined together, and the aper- 

 ture. Color, height of spire, proportionate length of aperture and 

 size of the shell are so variable that they have but little weight in 

 determining species, except in a few cases. 



The most reliable characteristics are, in the order here given, 

 texture, structure and sculpture ; in fact they are the only ones not 

 changed by environment. 



Texture relates to the weight and to the fineness and coarseness of 

 the layers of which the shell is made. The viscid matter secreted by 

 the mantle is deposited on the edge of the shell as the snail grows, 



