EXPLANATION OP PLATES. 



All specimens not otherwise described are in the collection of the Museum of 

 Comparative Zoology, and all not mentioned as "casts" have the shell present, either 

 in part or as a whole. Paul Roetter drew the figures in outline by measurement; the 

 author redrew all the specimens using these outlines, but testing the accuracy of the 

 measurements before they were finally placed on stone. The outlines in the Summary 

 Plates were sketched by the author, and redrawn by Miss Pierson. Unless otherwise 

 specified, the figures are approximately of natural size, although the process of reducing 

 by photography from the enlarged drawings has introduced some slight deviations from 

 the measured diameters of the originals. 



