INTRODUCTION. 



About ten years ago the writer pointed out the existence 

 of three great groups in the monotremate land snails, based 

 upon the structure of the pallial organs: SIGMURETHRA, 

 HETERURETHRA and ORTHURETHRA. The first group includes 

 a very large majority of the land snails now existing, which 

 have been described in volumes I to XVIII of this work. 

 The Heterurethra are chiefly represented by the genus Sue- 

 cinea. Finally the group Orthurethra is made up of a series 

 of families which retain primitive pallial structures like the 

 Basommatophora. They are the scattered descendants of 

 snails of long ago of the first land-snail fauna. With the 

 rise of the Sigmurethra, these primitive snails declined. On 

 the continents they take a minor role, and seem to exist by 

 grace of their insignificance (Pupillidcz, Valloniida, Ferus- 

 sacida), though one family, the Enidce, contains snails of 

 moderate size. On the Pacific islands, where the Sigmurethra 

 are represented by only a few low forms, Orthurethra are 

 still dominant. In this island world the Partulidce, Amastrida, 

 Ackatmellida and Tornatellinidce comprise the most con- 

 spicuous elements of the snail-fauna. 



The following families are now comprised in the group 

 Orthurethra : 



Valloniidae (See Vol. VIII, p. 247, and IX, p. 282). 

 Holarctic. 



Enidae (Buliminidce of authors). Pala&arctic, Oriental, 

 African. 



Pupillidse (Pupida of authors). World- wide. 



Partulidae (this volume, p. 155). Polynesia, Melanesia, 

 Micronesia. 



(vii) 



