LECTURES ON MOLLUSCA. 55 



shaped than is usual in the rasping tribes, and furnished with very 

 minute serrations. 



Already several differences have been pointed out among the animals 

 of this tribe, which may or may not be confirmed. Some of the groups 

 may hereafter be removed to other families. The principal genera are 

 as follows : The true Rissoce are somewhat pupiform in shape, with a 

 thickened lip, slightly pinched at the pillar, and a thin, slightly- 

 spiral operculum. In Cingula, the mouth is sharp and melanoid, 

 with flattened whirls. Alvania has the whirls round and is generally 

 sculptured ; the mouth also is round, with thickened lip. In Ris- 

 soina, which pretty much takes the place of Rissoa in tropical climates, 

 the shell is generally ridged, and the mouth thickened, produced in 

 front, with a strong pinch at the pillar : the operculum has a tooth at 

 the side, as in Nerita. Barleia has the shape of Rissoa, with an 

 annular operculum armed with an internal stump. Skenea is flat like 

 a Planorbis, with a round mouth and many-whirled operculum. Some 

 forms go to the opposite extreme, and are shaped like Turritella. They 

 have been supposed till lately to belong to Adis. The shells of this 

 group may always be known from the Pyramidellids by the point of 

 the spire being regular, not reversed. The Hydrobias live in brackish 

 water, 'in immense multitudes. The Nematuras, which float under 

 dead leaves in the rivers of the East, are like Hydrobia with a curi- 

 ously contracted aperture. The relations of Amnicola have not yet 

 been clearly made out, though the creatures swarm in the fresh waters 

 of North America. In shape they are intermediate between Bithinia 

 and Valvata ; but are known from both by the operculum, which is 

 spiral, with few whirls. 



Family LITORINIDJE. (Periwinkles.) 



The Periwinkles are formed for sea-shore life, and are destined to 

 scrape off and consume the various kinds of marine vegetation. They 

 abound everywhere except on sandy beaches, and each species has its 

 appropriate level in relation to the tide. Some are found at extreme 

 low water ; some at the ordinary high tide ; some where only the 

 spring tides reach them ; and a few where they are never covered with 

 water except in storms. Some crawl up the mangroves on the shore, 

 and some have been found walking on trees half a mile from the sea. 

 The ordinary Periwinkles have one very large gill in numerous plates 

 lying across the inner surface of the mantle. They have horny jaws, 

 and a thin spiral operculum, generally of few whirls. In shape some 

 of the shells resemble Turbos, and some Troclmses : but they may 

 always be distinguished by their want of pearly lustre. The Litorina 

 litoria is a favorite article of food with poor people in English cities ; 

 but the L. rudis, which inhabits a higher zone and brings forth its 

 young with a hard formed shell, is left to enjoy its native rocks. The 

 tongue is two inches long ; and the creature walks first on one side 

 of the foot, then on the other. There is a fold in the mantle pre- 

 senting an approach to the breathing pipe of the Whelks. There are 

 some river species, of Naticoid shape, which live on stones below water 

 in the Danube and La Plata. They are called Lithoglyplius . The 



