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THE AMERICAN NATURALIST. [Vol. XXXVII. 



holes in any two sections will not correspond exactly, as the 

 planes of the sections are not made accurately at the same angu- 

 lar distance from the origin of the spiral, but it can easily be 

 seen which holes are of about the same size. These openings 

 are, of course, the places where the lumen of the shell lies in the 

 plane of the section and serve as a convenient index of erosion. 

 In each of the older shells we can see the outline of the younger 

 stages. The line a-b in each section represents the lower bound- 

 ary of the stage corresponding to Fig. i. 



The shell represented by Fig. i is about 3 mm. long and 2.5 

 mm. broad, and is the smallest one that we were able to cut. 



On the body-whorl were three or 

 four ribs somewhat darker in color 

 than the rest of the shell. This 

 marking extended into the next 

 whorl, but toward the apex of the 

 acute spire the shell was smooth. 

 9 In the section the lumen of the 



leaving eight openings in the sec- 

 tion, and at the apex there is an 

 indication of another half turn of 

 the spiral. Each half turn of the 



is numbered consecutively, begin- 



Microscopic examination of the section, Fig. 1, shows that the 



a pearly white color with fine darker lines of growth or fracture 



is colored. Around each opening of the section the boundary 

 of the primary shell is clearly defined by a very fine hut per- 

 fectly distinct dark line. Within this boundary the lumen of the 



rial that we may call secondary shell. This may be white and 



