No. 435 ] SHELL OF LITTORLXA UTOREA. 173 



could be shown that the amount of erosion was so slight as to 

 be negligible. It occurred to us that possibly the amount of 

 erosion, on the contrary, might be even greater than would be 

 indicated by the condition of the surface. If the loss of mate- 

 rial from the surface should be balanced by the deposition of 

 new shell within the cavity, this might mask the erosion to such 

 an extent as to make it impossible to determine from the appear- 

 ance of the surface whether any given shell was much or little 

 eroded. With such an uncertainty any results obtained by the 

 comparison of the apexes of old shells with those of young ones 

 would be utterly worthless. 



We proceeded, therefore, to determine the amount of erosion 

 in shells of various sizes from the smallest to the largest. The 

 method employed was the study of thin sections made through 

 the apex of the shell in the plane of the columella. 



The sections were made in the following manner: — The 

 soft parts having been removed, the rounded part of the shell 

 was first ground from each side on an ordinary grind-stone used 

 for the sharpening of instruments. A slice was thus obtained 

 through the middle of the shell. One side of this was ground 

 with emery on a grinding machine used for making sections of 

 minerals 1 until a surface passing through the middle of the 

 columella was obtained. This surface was made perfectly even 

 by polishing with a paste of diatomaceous earth on a ground 

 glass plate. The polished side was then cemented to a glass 

 plate with Canada balsam, after which the other side was ground 

 down and polished in the same way until the section was suffi- 

 ciently thin. The section was then cleaned and mounted in 

 Canada balsam in the usual manner under a cover glass. 



In this way about a dozen slides were made. Figs. 1, 2, 3, 

 and 4 are camera lucida drawings of sections of four shells of 

 different sizes. Thus we have sections of two young shells, Figs. 

 1 and 2, a shell which is not quite fully grown, Fig. 3, and one 

 of full size, Fig. 4. The openings in Fig. 1 are numbered, 

 beginning at the top, and the corresponding openings in the 

 other sections are given the same numbers. Of course, the 



1 The authors are much indebted to Dr. C. H. Warren for helpful suggestions 



