220 LAND AND FRESH-WATER MOLLUSKS. 
OPERCULATED LAND SHELLS. 
ORDER OPERCULATA. 
THE Operculated Snails closely resemble the 
plant-eating gasteropods, as Bithinia and the 
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 
Helicide ; 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 
CycLostomaA ELEGANS — (the LHlegant Cirele- 
mouthed Shell) (Pl. XI., fig. 140).—The shell is 
