1 7 6 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



measure of the amount of erosion that has taken place since the 

 time when the wavy line was covered by the present body whorl. 

 Similar wavy lines are seen below openings 8 and 7 and there 

 the differences are greater, showing an increased amount of 

 erosion toward the apex, and the loss by erosion is still greater 

 at openings 6 and 5 where the secondary shell is exposed at the 

 surface. 



The same thing is shown more clearly in Fig. 3, a normal, 

 nearly full grown shell about 15 mm. long. Between openings 



1 1 and 1 3 there is a wavy line that forms a continuous curve 

 with the surface of the shell at 11. Here the erosion has pro- 

 ceeded only so far as to smooth off the ribs, while below openings 

 10 and 9 the wavy lines end abruptly almost at right angles to 

 the surface, and the exposed secondary shell at these openings 

 furnishes further evidence of erosion. At opening 9 the second- 

 ary shell itself is entirely removed on the outer side. But the 

 lumen of the shell is prevented from opening to the exterior by 

 a thick layer of tertiary shell, which has been deposited within 



