GUNDLACHIA. 235 
the earlier part of its life is spent, being 
undistinguishable 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) 
