ANALYTICAL KEY. XV11 



Shell ovately turreted, of six whorls, the last globosely inflated L, stagnalis. 



Shell oblong-ovate, of five to six rough convex whorls — palustris. 



Shell smaller, acuminately ovate, of six rough convex whorls — truncatula. 



Shell small, elongately turreted, of eight closely coiled whorls — glabra. 



Shell globose, extremely thin, of three transparent inflated whorls — glutinosa. 



Shell truncately ovate, thin, of three whorls, spire immersed — involuta. 



Carrying a limpet-shaped shell. Tentacles short, slenderly triangular ANCYLUS. 



Shell rotundately ovate, vertex posterior, incurved to the right — fluviatilis. 



Shell oblong-ovate, depressed, vertex more central, incurved to the left ... — lacustris. 



Opercidated. Respiring air. 

 Head produced, into a ringed proboscis. Tentacles bristle-like. 



Carrying a solid turbinated shell. Operculum shelly, spiral CYCLOSTOMA. 



Shell conically ovate, of five spirally corded whorls — elegans. 



Carrying a thin minute elongated shell. Operculum horny, subspiral ACME. 



Shell of six transparent horny whorls, mostly linearly striated — lineata. 



Opercidated. Sespiring water. 



Head produced into a ringed muzzle. Tentacles various. 



Tentacle and eye-stalk united in one, with the eye at their summit ASSI11INEA. 



Shell conical, of six sloping whorls. Operculum horny, few-whorled — Grayana. 



Carrying a turbinated shell with the aperture entire. 



Tentacles filiform, eyes on sessile swellings at their base BYTHINIA. 



Shell very small, of six whorls. Operculum horny, two-whorled — similis. 



Shell of five ventricose whorls. Operculum subtestaceous, concentric ... — tentacidata. 



Shell of five scalariform whorls. Operculum subtestaceous, concentric... — Leachii. 



Tentacles cylindrical, eyes on short stalks. Neck lobed on each side PALUDINA. 



Shell scalariformly turbinated of five to six whorls, umbilicated — contecta. 



Shell obtusely turbinated, of four to five whorls, scarcely umbilicated — vivipara. 



Tentacles cylindrical, eyes at inner base. Branchiae an external plume VALVATA. 



Shell small, of five globose whorls. Operculum concentric — piscinalis. 



Shell minute, of four discoid whorls. Operculum concentric — cristata. 



Eyes on slender stalks detached from the tentacles. 



Carrying an obliquely ovate shell. Operculum paucispiral, hooked NEJZITINA. 



Shell rather solid, of three whorls, generally painted with network — fluviatilis. 



MOLLUSKS WITHOUT HEAD. RESPIRING WATER. 



One pair of adductor muscles. 



Bearing an oblong fan-shaped shell. Affixed by a byssus. 



Mantle closed, except for the pedal, branchial, and excretory orifices DREISSENA. 



Shell triangularly trapezoid, very gibbous, painted with olive waves — polymorpha. 



b 



