In the present volume the important and difficult of group Doco- 

 glossate Gastropods, the Limpets, is monographed, and in addition a 

 small group not heretofore included. The material studied in 

 the families Acmceidce and PatellidcK is exceptionally extensive. A 

 large number of forms are herein for the first time figured and 

 adequately described. The value of the work has been enhanced 

 by the liberality of Dr. W. H. Ball, of Washington, who placed at 

 the author's disposal for study the magnificent collection of the 

 Smithsonian Institution, a collection especially rich in species from 

 the west coast of America, and containing the types of species 

 described by GOULD, CARPENTER and DALL, many of which have 

 not before been figured. No effort has been spared to make the 

 synonymy and references complete and reliable ; and it is hoped 

 that conchologists will find the labor of classifying their collections 

 of these intricate groups decidedly lightened. 



Philadelphia, June, 1891. H. A. P. 



