
OUR COMMON FRESH-WATER SHELLS. 531 
sketch will be attempted. In one sense the papers will be a 
compilation from the treatises of Prime, Binney, Bland, and 
others, published by the Smithsonian Institution, through 
whose liberality we are enabled: to illustrate this and the 
papers which are to follow. 
A shell common in most of the streams and ponds of New 
England, a figure of which is here given, Fig. 78, belongs to 
a group of mollusks that is distributed 
throughout the northern hemisphere. 
They are usually found in muddy streams 
or ponds, either grovelling an inch or so 
in the mud or among roots, or crawling 
along over the sand. 
The creeping disk is quite long and gy 
broad. The little snout, on each side of 
which may be seen the tentacles, with 
eyes at their bases, projects beyond the margin of the shell 
in front, while behind the shell, and attached to the upper 
part of the tail may be seen a semi-circular corneous plate 
called the operculum, Fig. 19. In Pl. 9, fig. 2, another spe- 
cies is represented in the attitude of crawling, showing the 
position of the operculum. When the animal retires within 
its shell the head and forward part of the foot disappears 
first, followed by the tail with the operculum, which rig. 7. 
answers as a lid, or door to close the aperture of 
the shell. In Figs. 1 and 3 of the plate, the appear- / 
ance of the operculum is shown within the aperture (45 
of the shell. As the shell increases in size, by the 
addition of tiny particles around the margin of the aper- 
ture, the operculum inereases likewise by the addition of 
the corneous substance around its margin, and the little 
Concentric furrows seen in the figure of the operculum 
indicate its successive rates of increase. Most marine 
Gasteropods (the name of the class to which all those mol- 
lusks belong that have a broad creeping disk) are furnished 


