220 LAND AND ERESH-WATER MOLLUSES. 
OPERCULATED LAND SHELLS. 
Order Operculata. 
The Operculated Snails closely resemble the 
plant-eating gasteropods, as Bithinia and tho^ 
periwinkle; and chiefly differ from them in their 
habitats and the medium they respire. They 
have a long muzzle, two slender tentacles, con¬ 
tractile, but *not retractile, like those of the 
Helicidce; the eyes are sessile. The lingual ribbon 
differs very widely in the aspect of its teeth from 
the land-snails, and is narrow. The teeth are 
large; three on each side of a central tooth, and 
thus they very much resemble those of the fluvia- 
tile gasteropods. The sexes are distinct; the 
aperture of the shell is closed by an operculum. 
The order contains two families, each with a 
single representative species in Great Britain; 
they may be distinguished as follows :—• 
Shell oval; operculum shelly. Cyclostoma elegans 
Shell cylindrical; operculum horny . Acme fusca 
Cyclostoma elegans — (i the Elegant Circle- 
mouthed Shell) (PI. XI., fig. 140).—The shell is 
