INTRODUCTION. XI 



onic. The nepionic shell-record is always brief in pulmonates 

 the fractional part of a whorl but in many genera it is 

 legible. Developmentally important, since it includes the 

 decline and disappearance of embryonic structures, such as 

 the podocyst, and rapid development of kidney and other 

 neanic structures ; terminates with birth. 



Neanic. Youth, up to the completion of the shell, which is 

 usually coincident with the time of reproductive activity. 



Ephebic. Maturity. 



Gerontic. Old age. 



Each of these stages may be subdivided by use of the pre- 

 fixes ana-, meta-, para-. Thus ananeanic signifies an early 

 neanic substage; metaneanic, mid-neanic; and paraneanic, a 

 late neanic substage, or almost full grown. 



It will be noted that many Tornatellinid snails are phylo- 

 gerontic, or in the old age of the race, shown by the culmina- 

 tion of structural differentiation in the neanic stage, with 

 simplification in the later stages. Many instances are noted 

 under Lamellina, Tonwtellides and other genera. It is an 

 indication that such genera are geologically old. This con- 

 clusion is supported by the facts of distribution, for several 

 of the genera range over islands scattered from side to side 

 of the Pacific, where the facilities for migration of land ani- 

 mals must be the least favorable. 





