X INTRODUCTION. 



tribution of the Achatinellidcu, being absent from Kauai. The 

 Hawaiian archipelago has by far the greatest number of 

 genera and species, having representatives of all the genera 

 except Lamellovum; but Polynesia, Micronesia and Melanesia 

 have not been so fully explored. 



What we know of the distribution and structure of Torna- 

 tellinid snails suggests that the group is of great antiquity. 

 It has probably had since Palaeozoic times to acquire its pres- 

 ent distribution. Yet the presence of closely related species 

 on separate island groups indicates that some emigration has 

 been in progress up to recent times, whether by wind or flot- 

 sam, or both, is uncertain. A few of the species which occur 

 on low islands, and near the shore of others, have in all prob- 

 ability been transported from island to island by human 

 agency, as they cling to leaves and stems of plants and other 

 objects, and might be picked up in any cocoanut grove. 



Large accessions to the list of species now known may be 

 expected from the Hawaiian Islands, Polynesia, Micronesia 

 and Melanesia, particularly from the high islands. Eapa and 

 Juan Fernandez are points of especial interest for further 

 investigation. 



TERMINOLOGY OP THE STAGES OF GROWTH. 



The shells of most TornatelUnidce change remarkably in 

 structure during the development of the individual. The 

 characters of adolescent stages often indicate relationship not 

 expressed by the adult stage. In the discussion of these stages 

 we have used the well-known modified Hyattian terminology 

 of the stages of growth and decline, as follows : 



Oonic. Egg, up to maturation. 



Embryonic. From fertilization to the culmination of podo- 

 cyst, primitive kidney and other specially embryonic struc- 

 tures. As the succeeding (nepionic) stage is not distinguish- 

 able in shells of Tornatellinidae, "embryonic" is here prac- 

 tically used to cover the stage ending with birth or escape 

 from the egg capsule. 



Nepionic. Babyhood. Scarcely or not recognizable in the 

 shell, in this family, and in practice included in the embry- 



