GUNDLACHIA. 235 



the earlier part of its life is spent, being 

 undistinguislia])le in form from, the shells of 

 that genus. It is probable that it passes the 

 first summer and autumn of its existence in 

 this smaller shell, and that the septum, which 

 afterwards partially closes its aperture, is 

 formed during the period of inaction which 

 ensues during the winter. The septum would 

 in some degree serve as a protection to the 

 mollusk during this period, in the same way 

 as the epiphragm of the Helices. In the fol- 

 lowing spring the additional growth of shell 

 commences, but in the winter it would proba- 

 bly present the appearance of an Ancylus 

 with two-thirds of its aperture closed by a 

 septum, leaving but a small opening for the 

 egress of the foot of the animal. 



(29) 



