12 INTRODUCTION 



and from the Eleventh Report of the State Geologist of Indiana, which 

 may be consulted for a more ample representation of the adult characters 

 of the species occurring at Waldron. 



The drawings on plates i to 7 have been reproduced on stone, in a 

 most satisfactory manner, by Mr. Philip Ast, and we wish to express our 

 appreciation of the skill and labor he has bestowed upon the work. The 

 illustrations given on plate 8 were made from photographic reproductions 

 of the actual series of specimens, and, although not serviceable for purposes 

 of detailed study, show distinctly the nature of the material used and the 

 almost insensible gradations obtained, representing the life-history of these 

 species. The same completeness of material is furnished by the majority 

 of forms described in the following pages. 



The arrangement of the subject-matter in the discussions of the species 

 may not seem to be in accordance with the usual method employed in trac- 

 ing the life-history of organisms. In this case we are dealing with fossil 

 organisms and, in order to insure accuracy of results, it is necessary to 

 begin with the known and established facts and gradually descend to 

 to minute and strange forms, thereby connecting the extremes of growth. 

 Under the caption, " Developmental Changes," we have, however, endeav- 

 ored to trace the history of each feature of the shell, from its inception to 

 maturity. 



