vm 



INTRODUCTION. 



with typically five laminae or teeth within : angular, parietal, 

 columellar, upper and lower palatal, no marginal teeth on the 

 peristome. The shape of the shell is changed in some genera, 

 but not especially in the direction of other families of 

 Orthurethra. From the strict homologies of the five primary 

 teeth throughout the family it seems likely that the original 

 pupillid stock possessed them; but in all of the derivative 

 lines of descent (subfamilies) there are forms with the full 

 tooth formula, together with others, in which there has been 

 simplification or total loss of teeth. 



In the collateral family Valloniidae there is no evidence of 

 a five-plicate aperture in any of the living or fossil forms, 

 though two of the leading genera are known in numerous 

 species from the Paleocene to the present time. The char- 

 acter of being toothless from the earliest appearance has not 

 I think, been sufficiently appreciated by those who would 

 unite the Valloniidae with Pupillidae or Strobilopsidae. The 

 only dentate genus, Spelaediscus, has teeth of a type wholly 

 different from Pupillidae, and which may reasonably be re- 

 garded as newly arisen in that genus. The Strobilopsidae 

 have a complicated system of internal laminae, already de- 

 veloped in the Eocene and persisting with little change to the 

 present. I have elsewhere mentioned the possibility of homo- 

 logizing the principal elements of this armature with pupillid 

 teeth. 



The interrelations of the subfamilies of Pupillidae, as I 

 understand them, may be represented in a diagram, thus : 



Gastrocoptinae Vertigininae 



Pupillinae 



Orculinae Nesopupinae 



Ancestral 

 Pupillidae- 



-Strobilopsidae 



Valloniidae- 



