CLASSES, ORDERS, ETC. 65 



large ; mouth oval, angulated above; umbilicus none ; operculum 

 oval, angulated above, closely fitting. Length J inch. Streams, 

 ditches, canals. B. tentaculata. lf> 



(b) Shell conoid, swollen towards the base, thin, subtransparent, 

 yellowish-horn coloured; whorls four to five, tumid, separated 

 by a deep suture ; mouth nearly circular, less angulated above 

 than in B. tentaculata ; umbilicus small, distinct ; operculum nearly 

 circular. Length J inch. Ditches, canals, and sluggish rivers. 

 More local than B. tentaculata. B. Leachii. 1 * 



4. Hydrobia. Operculum horny, thin ; eyes placed on 

 tubercles. 



(a) Shell ovoid, subopaque, thin, yellowish-horn coloured ; 

 whorls five to six, very convex ; sutures deep and grooved ; 

 umbilicus represented by a narrow chink ; mouth oval ; operculum 

 oval, nucleus lateral. In ditches between Greenwich and Wool- 

 wich which are flooded by the tide. H. similis. 



() Shell a lengthened cone, thin, glossy, yellowish-horn 

 coloured ; whorls six to seven, convex ; sutures not grooved ; 

 mouth oval; umbilicus smaller than in If. similis. 



H. ventrosa. 11 



Family III. Valvatidae. 



Shell conoid, more or less depressed ; mouth circular ; oper- 

 culum horny. Body spiral, with two tentacles, provided with a 

 long plume-like gill which is protruded when the animal is 

 crawling; eyes situated on the inner side of the base of the 

 tentacles. 



5. Valvata. 



(a) Shell globular, thin, solid, light horn-coloured, whorls five 

 to six, rounded, body-whorl very large ; spire obtuse, compressed ; 



15 v. albidct) white ; v. fulva, tawny, glossy ; v. ventricosa, more tumid, 

 globular-conical in shape, white ; v. zonata, reddish with one or more white 

 lines or bands ; v. producta, less tumid, in shape an elongated cone ; v. 

 excavata, whorls more rounded, suture much deeper ; m. decollalum, with the 

 apex decollated ; v. pallida, shell very pale horn coloured ; m. scalariformc, spire 

 scalaria. 



16 . v. albida, white; v. elongata, smaller, spire more produced. 



17 v. pellucida, clear white, nearly transparent ; v. minor, smaller, spire 

 shorter ; m. decollatum, spire decollated. 



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