No. 435.] SHELL OF LITTORINA LLTOREA. 



the secondary and exhibits lines of growth parallel to its own 

 surface and unconformable with those of the secondary shell. 

 All of the openings above 13 in this specimen show a layer of 

 tertiary shell, which becomes progressively thicker toward the 

 apex. The numbers assigned to the openings in this specimen 

 are obtained by comparison with Fig. 2, and, if the reasoning 



applied to that figure will hold for this one, the first five openings, 

 that is two whorls and a half, have disappeared entirely. 



Fig. 4 is from one of the largest of the specimens collected, 

 about 20 mm. long. But somewhat larger shells are not rare. 

 To the naked eye the rounded apex and white color of the apical 

 region in this specimen indicated a considerable but not excessive 

 amount of erosion. The difference in color is shown by the 

 section to be due to the exposure of a large amount of tertiary 



