68 LAND AND FRESH-WATER SHELLS. 



(h) Shell nearly flat above, rather convex below, yellowish-horn 

 coloured, thin, glossy ; whorls five to six, the outer whorl being 

 sharply keeled in the middle line of its periphery; mouth obliquely 

 oval, somewhat angular ; umbilicus small. Diameter \ inch. 

 Stagnant waters and sluggish rivers. Local. P. carinatus^ 



(i) Shell slightly concave above, flattish, with a central con- 

 cavity below, striolate, brown-horn coloured ; whorls five to six, 

 rapidly enlarging, with the periphery strongly keeled below ; 

 suture deep ; mouth rhombic, rounded in front, often ribbed in- 

 ternally ; umbilicus large, shallow. Diameter |ths inch Ponds, 

 canals, slow rivers, and ditches. P. complanatus?* 



(j) Shell reddish-brown, "very tumid, glossy, nearly opaque; 

 whorls five to six, rounded above and below ; periphery not 

 keeled ; mouth semilunar ; umbilicus broad, shallow. Diameter 

 | to i inch. Canals, streams, marshes, and ponds. Somewhat 

 local. P. corneus'^ 



(k) Shell nearly flat above with a deep concavity in the middle, 

 very convex below, solid, opaque, yellowish or brown-horn colour ; 

 whorls eight, compressed; suture deep; mouth crescent-shaped, 

 very narrow ; umbilicus large, deep. Diameter T 2 ^ths inch. 

 Ponds, ditches, and lakes. P. contortus?* 



(/) Shell flattish, dull, very convex below ; whorls two to two 

 and a half, with the periphery of the outer whorl angulated ; 

 mouth large, squarish, and much expanded ; umbilicus abruptly 

 contracted, small, deep. Diameter -^th inch. In canals round 

 Manchester and Burnley. Imported. P. dilatatus. 



7. Physa. Shell spiral, thin, polished ; spire smistral, non- 

 operculated. Animal with two long tentacles, with the eyes at 

 their base ; mantle very large so as to cover part of the shell. 



(a) Shell fusiform, thin, yellowish or reddish-horn coloured, 

 glossy, semi-transparent ; whorls six to seven, rounded ; suture 



25 v. albida. pellucid white ; v. disciformis, yellowish, flatter, thinner, keel 

 more prominent and sharp. 



26 v. albina, whitish or colourless ; m. terebrum, whorls dislocated from one 

 another, and elevated into a spiral cone ; v. rkombea, smaller, more convex 

 above, with a deep concavity below, keel blunt. 



27 v. albina y perfectly white. 



28 v. albida, nearly white ; v. excavata, much depressed, sunken above. 



