INTRODUCTION 



The division of the Orthurethra into families seems to be 

 largely a matter of expediency. Hugh Watson (1920), after 

 showing that the various anatomical characters occur in a 

 great number of combinations, suggested that the entire series 

 be merged into three families, Achatinellidae, Partulidae and 

 Pupillidae, the latter with numerous subfamilies. Steenberg 

 (1925), with more analytical tendencies, proposed to recognize 

 16 families. The failure to find exclusive character-combin- 

 ations for families has concerned me since 1900, when the 

 snails composing the group Orthurethra were first brought 

 together. The division into ten families in this Manual had 

 its basis in (1) the recognition of features of the shell as im- 

 portant family characters, and (2) the idea that penial com- 

 plications are fundamental, apparently present in the group 

 as it was before breaking up into most of the existing families, 

 and thus inherited in nearly all of them. Forms with simple 

 genitalia are therefore relatively evolved and have arisen 

 de novo in various families — a view which has been already 

 either admitted or assumed by several authors. To believe 

 that similar penial complications have arisen in several 

 obviously distinct lines seems to stretch the probabilities of 

 homoplasy too far. 



Apertural teeth or lamellae in the shell appear to have 

 evolved after the initial divergence of the family stocks, as 

 they do not seem to be traceable to any common type. The 

 Pupillidae, Cochlicopidae, Tornatellinidae, Partulidae, etc. 

 apparently each evolved its special type of apertural arma- 

 ture independently. In some families, such as Pupillidae and 

 Tornatellinidae, this evolution appears to have been very 

 early, and many members now show degeneration of teeth. 

 In others, such as Partulidae and the genus Spelaeodiscus in 

 Valloniidae, the evolution of teeth seems to have been one of 

 the latest developments. 



The family Pupillidae and its subfamilies. — The Pupillidae 

 are characterized by the oval to eylindric shape of the shell, 



(vii) 



