PREFACE 11 



In the preparation of the body of the report new definitions of all of the 

 species have been written so that the usage of terms is uniform through- 

 out. In the definitions, the usual size of the members of any species has 

 been expressed in the relative terms large, small, etc., these terms being 

 used on the arbitrary basis that a specimen having a diameter of about 25 

 mm. or 1 inch, is medium sized. Specimens smaller than this are below 

 medium size or small, while specimens of larger dimensions are above 

 medium size or large. Besides these general dimensions, accurate meas- 

 urements of one or more specimens have always been given. The usage of 

 the terms cast and mould is not entirely uniform among paleontological 

 writers. In the present work the term internal cast, or cast of the in- 

 terior of the shell, is applied to the hardened matrix which has filled the 

 interior of the shell or valve, the cavity of the shell or valve being as- 

 sumed to be the mould in which such a cast was formed. 



All the illustrations on the accompanying plates are reproduced from 

 photographs taken from the actual specimens. During the earlier part of 

 the work I was materially assisted in the preparation of photographs by 

 Mr. J. M. Jessup, and I wish here to acknowledge his very efficient aid in 

 this part of the work. 



STUART WELLER. 

 Chicago, September 1, 1913. 



