species, while organisms apparently quite diverse may be treated as 

 varieties, simply because they are united by a series of intermediate 

 forms. 



In cases where I have no specimen of a species, I have given 

 a translation of the original diagnosis, followed by the name of the 

 author from whom it is taken, together with any additional inform- 

 ation I have been able to obtain concerning it. 



Anatomical details have been omitted because the preceding vol- 

 umes of Helix lack them. Under these circumstances it is more 

 convenient to present all of this part of the subject in one place. 

 The final volume treating of Helices will therefore contain besides 

 other matter, a resume of the anatomy of each subgenus of the 

 Helices. Collectors having either fresh or alcoholic material for 

 sale or exchange are invited to correspond with the author. 



In the preparation of the present volume the author has been 

 peculiarly fortunate in the opportunities for study afforded by a 

 collection of AVest Indian Land shells perhaps the largest in exist- 

 ence ; the extensive collections of ROBERT SWIFT and of A. D. 

 BROWN having in recent years been united to the already large suites 

 received by the Academy from POEY, ARANGO, BLAND, WRIGHT, 

 REDFIELD, and other workers in this fauna. 



H. A. P. 



PHILADELPHIA, March, 1889. 



