PUPILLIDAE AND RELATED GROUPS. 



193 



shell armature. [The Pyramidulinae and the Pleurodiseinae 

 are exceptions to this general rule and may represent the 

 most primitive conditions, before the evolution of complex 

 genitalia.] 



From the Valloniinae, a line (or progressive tendency), 

 usually with more calcareous shells, terminates in the Gastro- 

 coptinae, which have simplified male genitalia and usually 

 develop complex shell armature, but which retain, except in 

 Modicella, a radula similar to that in Vallonia. On the other 

 hand, from the Acanthinulinae, a divergent line, usually with 

 more epidermal shells, terminates in the Vertigininae, which 

 also have simplified male genitalia and usually develop shell 

 teeth, but which tend to produce interstitial cusps (and 

 entocones) throughout the radula. This idea may be repre- 

 sented graphically as in fig. 10. 



PUPXILIHAC 



ViLLLORIIIIAI VfRTISISISAt 



Fig. 10. 



An alternative hypothesis would be to consider complexity 

 of shell armature and simplicity of genitalia as the convergent 

 characters. This would bring the Vertigininae and the Gastro- 

 coptinae together but would separate the Valloniinae (s. s.) 

 from the rest of the Valloniinae (sens, lat.) as the terminals 

 of the two divergent lines. 



Strobilopsidae. 



The only reason for the retention of this group as a family 

 distinct from the Valloniinae lies in its shell characters, es- 



