139 



LIMNiEAD^. 



The fresh-water pulmoniferous snails constitute a very 

 natural family, the animals of all bearing a great similarity 

 to each other, and being similarly organized. All have 

 short broad snouts, and two tentacula of considerable size, 

 either triangular or subulate in shape, with eyes placed 

 at their inner bases. Their tongues are armed with rows 

 of numerous quadrate hooked denticles. Their shells are 

 very variously shaped, spiral, and turreted, dextral or 

 sinistral, discoid and even patelliform. They have no 

 operculum. They live in lakes, ponds, pools, ditches and, 

 though not so abundantly, in rivers ; occasionally they are 

 found in brackish waters. They crawl on the mud and 

 stones at the bottom, or on water plants, and in warm 

 sunny weather ascend to the surface, and creep, as it were, 

 reversed on the surface of the water, as if the film of water 

 immediately in contact with the air was in a different 

 condition from the fluid beneath, and served as a floor or 

 ceiling, along which these mollusks progress. They lay 

 their eggs in consistent transparent gelatinous masses on 

 the leaves and stems of water plants, or on stones. 



The characteristic forms of the generic types of this 

 family have undergone very little change in the course of 

 time since the earliest appearance of the group as yet 

 traced. We find Limnaada in the fresh- water strata of 

 the oolitic epoch strikingly resembling those which inhabit 

 the pools and ditches of Britain at the present day. 



