UNIVALVES. 
45 
sometimes open or umbilicated: this perforation 
may be a mere chink (cl), or it may be filled by a 
shelly deposit in the adult, as in many land 
shells. The last turn of the shell or body whorl 
is usually large. The base of the shell is the end 
opposite to the apex (e); and the aperture is 
entire (/), that is, not notched or produced into 
a canal. The margin of the aperture is called the 
peristome , which in Paludina is continued all 
round; in the Rock Snail it becomes so in the 
adult; more frequently, among the shells of the 
next class, the peristome is incomplete, the left 
side of the aperture being formed by the body 
whorl; the right side of the aperture is formed 
by the outer lip, the left side by the inner, or 
columella lip. The outer lip is thin, not thick¬ 
ened or reflected, as in the majority of the land 
shells; but in immature shells of them it is 
always thin and sharp. The aperture of the shell 
is upon the right-hand side, when the shell is 
viewed in a vertical position, with the aperture 
directed toward you: this is the general rule, 
but in a few of the shells of the air-breathing 
snails the shell is twisted in the opposite direc¬ 
tion : in the former case the shell is said to be 
dextral, and in the latter, reversed or sinistral. 
Lines of colour or sculpture are termed spiral or 
longitudinal when they proceed from the apex 
around the whorls; radiating, if they extend across 
