INTRODUCTION 



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Atrypa reticularis, Linnseus, 

 Zygospira minima. Hall, 

 Ccelospira disparilis. Hall, . 

 Retzia evax. Hall, 

 Retzia sobrina, sp. n. 

 *Nucleospira pisiformis. Hall, 

 Meristella rectirostra. Hall, 

 Meristina tiitida. Hall, . 

 Whitfieldia maria. Hall, . 

 Spirifer eudora. Hall, . 

 Spirifer crispus, Hisinger, 

 Spirifer crispus, var. simplex. Hall, 

 Spirifer radiatus, Sowerby, 

 Spirifer bicostatus, var. petilus. Hall, 



Young shells abundant. 

 No young -shells obtained. 

 Young shells common. 

 Young shells very abundant. 

 Young shells not rare. 



Full series, showing development. 

 Numerous inchoate specimens. 

 Incomplete series showing development. 

 No young shells obtained. 

 Full series, showing development. 



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 enlarged, 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 structure, both the develop- 

 mental 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, we have sometimes 

 enlarged the earlier phases to a size corresponding with the same structure 

 in the mature form, or have increased all on a uniform scale, so that both 

 the particular characters and their comparative size are presented. 



The enlarged drawings have been made by the writers, principally 

 from the microscope ; the camera lucida was employed to ensure accuracy 

 in outline. The illustrations of the mature specimens are largely taken 

 from the Twenty-eighth Annual Report of the New York State Museum 



*The mature characters of this species are assumed so early, that the youngest forms ob- 

 served show no important differences from the adult. On this account, no discussion of its 

 characters is given in the ensuing pages. 



