68 LAND AND FRESH-WATER SHELLS. 



brownish-horn coloured, transparent ; whorls six to eight, the 

 outer whorl sharply keeled on its lower margin ; mouth rhombic, 

 compressed; umbilicus large, shallow. Diameter fths inch. 

 Shallow and stagnant waters. Moderately common. P. vortex.^ 



(g) 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.^ 



(ti) 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 fths inch. Ponds, 

 canals, slow rivers, and ditches. P. umbilicatus. m 



(i) 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. comeus? 1 



(j) 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 jVris inch. 

 Ponds, ditches, and lakes. P. contortus! 



(k) 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 T ^th inch. In canals round 

 Manchester and Burnley. Imported. P. di/atafus. 



24 v. compressa, thinner, flatter, keel more distinct, and sharper, and placed 

 nearly in the middle of periphery. 



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

 more prominent and sharp. 



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

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

 above, with a deep concavity below, keel blunt. 



27 v. albina, perfectly white. 



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



