LECTURES ON MOLLUSCA. 205 



again, having first anchored themselves to it by a thread. The oper- 

 culum is said to have many whirls. 



Family Valvatid.ze. 



Another aberrant family consists of little shells looking like fresh- 

 water Cyclostomas. They have perfectly round mouths, and the shell 

 is sometimes a little raised, sometimes quite flat. Alone of all the 

 Prosobranchiate Gasteropods, their gills are exposed to view; being 

 exserted, on the left side of the animal when walking, in the shape of 

 a very slender pinnate leaf. When the animal retires, the gill is 

 drawn into its cavity. The operculum is many-whirled. The Valvatce 

 live in rivers, lakes, and ditches in temperate regions of both the Old 

 and New World. As the V. tricarinata is extremely common in the 

 northern States, it is to be hoped that some naturalist will examine 

 whether the creature is hermaphrodite, as stated by Dr. Gray. If so, 

 this again is an anomaly in the Comb-gilled order. Shells not to be 

 distinguished generically from living Valvatas are found even in oolitic 

 strata, associated with Bithinice, Paludince, &c. It would appear that 

 the types of Molluscan life have not changed in fresh waters so much 

 as in the marine forms. 



Family Paludinid^. {River Snails.) 



The Paludince take the place of the Ampullarice in the temperate 

 regions; but the animal is much more like the Periwinkles. They 

 have a long, contractile muzzle; and neck- lappets, folded to make a 

 rudimentary breathing gutter. The eyes are on stumps at the base of 

 the tentacles. The Paludince are viviparous, the young being born 

 with a delicate shell of three whirls. The operculum is thin, and an- 

 nular as in Ampullarice. The tongue-ribbon is strong but slender; 

 the teeth not much bent, and very finely hooked. The creatures are 

 very sluggish, generally living imbedded in soft mud at tire bottom of 

 rivers or deep ditches. They live on decaying animal and vegetable 

 matter. The smaller species are oviparous, and have a shelly coat to 

 the operculum. They are called Bithinia, and have only one neck 

 lappet on the mantle. Among the mountain streams of Ceylon, some- 

 times at a height of six thousand feet, are found a group of shells 

 remarkable among fresh- water snails for their solidity. Their surface 

 is generally rough with knobs or ribs, and the point eroded by the 

 acid of the water. The last whirl is very spacious, as in the Ampulla- 

 rice, and is closed by an operculum increasing concentrically from the 

 margin, presenting a shape very similar to that of Purpura. They 

 have been erroneously described as Paludomus, and are now known 

 under the name of Tanalia. 



Family Melaniad^;. 



The Melanias are a tribe of fresh- water snails, abundant in all the 

 sub-tropical regions of the globe. In America they swarm in all the 

 southern regions, to the great delight of species -makers, who can at 

 anytime immortalize themselves by wading in some unsearched stream ; 



