BANDED WATER-SNAILS, PERIWINKLES, ETC. 635 



The AssiMiNiDJS are small, high-spired shells that live in brackish waters, 

 near the mouths of large rivers through Europe, Asia, and in the Antilles. 

 One species is still to be found in ditches beside the mouth of the Thames, 

 but is rapidly becoming extinct. 



The Ampullakiid^ dwell in tropical lakes and rivers. They have globular 

 shells, with greenish periostracum, and closed with a horny or shelly con- 

 centric operciilum. In some species the shell is sinistral. The animal of 

 AmpuUaria has a double respiratory chamber — one portion for breathing air, 

 the other for respiring water. 



The VALVATiD.as, in which there is but a single genus Valvata, are 

 restricted apparently to the northern hemisphere. Two species occur in 

 Britain. The shell is flat, or moderately spired, and furnished with a many- 

 whorled operculum fitting the circular mouth. 



The ViviPARiD^ have two common British representatives, known as the 

 banded water-snails. The shell is high-spired with rounded whorls, the 

 mouth is oval, and the operculum horny. 



The right tentacle of the males is shorter than the left, and has a 

 deformed look. The eggs are usually hatched within the parent. The 

 family is confined to the northern hemisphere, except in Africa, where it 

 extends to the head- waters of the Nile. 



The PAiUDESiRilsriDiE are miniature editions, so to speak, of the preceding; 

 but the tentacles are not modified. The operculum is calcareous in Bybhinia. 

 They are world-wide in their distribution, and there are several British 

 representatives. 



The RissoiiDjE are minute marine shells, very similar to the preceding ; 

 they are abundant everywhere. The LiTiopiDiE and Jeffketsiid^ are 

 closely allied to the Rissoiidte. 



The LiTTOKiNiD^, or Periwinkles, both the common one with its round, 

 banded shell and short, sharp spire, and the yellow one without any spire, 

 are too well known to need description. Some of the species have spiral 

 ridges on the shell. Pagodus has a conical shell with tubercles or spines. 

 The operculum in this family is horny and spiral. 



The FossAKiDiE are closely allied and very similar to the Littorinidse. 



The Melaniid^ and PLEUROCEKiDiE have as a rule very high-spired shells 

 covered with a brown periostracum ; the mouth is simple, and the spiral 

 operculum oval. The whorls are sometimes smooth, sometimes ornamented 

 with spiral ridges and lines of tubercles, or a row of spines near the suture. 

 They are exclusively fresh- water forms. 



The C^oiD^, sole genus Cacum, are strange little curved shells without any 

 markings. The young shell is coiled, but the creature, as it grows, breaks 

 off successive portions and blocks up the end with a shelly plate, so that the 

 whole shell is never found in the adult. It has a horny, spiral, circular 

 operculum. Two species occur in Britain. 



The TuRRiTEiLiDiE, or screw-shells, have long, slender, pointed shells of a 

 great many whorls, more or less keeled, or ornamented with spiral ridges ; 

 the aperture is small, the lip simple, and the operculum horny, many- 

 whorled, and covered with short hairs. The family has a world-wide 

 distribution. One species of Turritella is British. 



The Vermbtid^, when young, often have a neatly-wound spiral shell like 

 Turritella, but in the more mature and the adult state this straggles off into 

 an irregular circular tube much resembling those of certain marine worms. 

 The shells are attached to each other in masses or to other objects. The foot 



