
532 OUR COMMON FRESH-WATER SHELLS. 
with opercula, though they vary greatly in composition and 
shape. Some are strengthened by the addition of lime, and 
are quite. solid ; of this kind is the eye tone, so called ; some 
are claw-shaped, circular, or very irregular in form. 1n 
most species the operculum fits the aperture of the shell 
very closely; in others the operculum is rudimentary. ln 
Strombus, or the conch-shell, it is long and sharp, projecting 
some way beyond that portion of the foot to which it is 
attached, and the animal uses it by thrusting it into the sand, 
and then by a quick muscular contraction throwing its whole 
body forward. While most mollusks lay eggs, some in à 
glairy mass, as in the air-breathing water snails, or in à 
series of pods like the whelk, the group of which we are 
now treating, bring forth the young alive, but the young are 
simply hatched from the egg, before the egg leaves the 
parent; hence they are called ovoviviparous, On breaking 
open the shell of a female in spring time, the young ones 
may be found of various sizes within their globular eggs- 
The species figured above, and also in Pl. 9, fig. 11, is now 
known as Melantho decisa, and is the only species found in 
New England. The shell is quite solid, having four or five 
whorls; though the first two whorls, forming the tip of the 
shell, is KERA absent from erosion. In young specimens 4 
perfect one may be found ; but adult shells are always imper- 
fect, as shown in the figure. The color of the shell varies in 
being a light or dark green, and shiny. Within the aperture 
the shell is bluish white. 
Those who have the first volume of this magazine will 
recall the description there given of the tongue of a land 
snail, in which it was stated that the floor p the snail’s 
mouth was lined by a membrane covered with many rows 
of minute spurs, or teeth, and that the snail used this tongue 
in rasping its food. Now these minute teeth furnish admi- 
rable characters in the classification of these minor group? 
of mollusks, Thus the air breathing snails which have n9 
morum have the tongue lined with rows of very nU- 

