316 STUDIES IN EVOLUTION 



The method of illustration which has been adopted is one 

 which seems most readily to furnish a means for comparison 

 of characters. The embryonic shells are represented as en- 

 larged, usually to the size of an adult, and accompanying the 

 enlargements are natural-size representations of the final 

 result of normal growth. Where the mature forms have 

 been too minute to show satisfactorily the details of struc- 

 ture, both the developmental stages and full-grown shell 

 have been enlarged to a convenient size. Thus the incipient 

 stages and mature specific form are presented together. In the 

 delineation of special features, such as the hinge, the writers 

 have sometimes enlarged the earlier phases to a size corre- 

 sponding with the same structure in the mature form, or 

 have increased all on a uniform scale, so that both the par- 

 ticular characters and their comparative size are presented. 



The enlarged drawings have been made by the writers, 

 principally from the microscope; the camera-lucida was em- 

 ployed to insure accuracy in outline. The illustrations of 

 the mature specimens are largely taken from the Twenty- 

 eighth Annual Report of the New York State Museum 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 XV to XXI have been reproduced 

 on stone, in a most satisfactory manner, by Mr. Philip Ast, 

 and the writers wish to express their appreciation of the skill 

 and labor he has bestowed upon the work. The illustrations 

 given on Plate XXII were made from photographic repro- 

 ductions 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 



