CLASSES, ORDERS, ETC. 6"] 



(c) Shell quoit-shaped, thui, rather concave above, rather convex 

 below, not glossy, dull light brown or grey, subtransparent \ 

 whorls three, the cuter whorl bluntly and indistinctly keeled on 

 its periphery, and strongly marked with transverse ridges in line 

 of growth ; mouth oval ; umbilicus large. Diameter -j^g-th inch. 

 Ponds and ditches on aquatic plants. P. nautileus?^ 



{d) Shell convex above, concave below, thin, whitish, trans- 

 parent, finely striated longitudinally, and marked with fine close- 

 set raised circular striae which are clothed with deciduous bristles ; 

 whorls five ; mouth roundish-oval ; umbilicus large. Diameter 

 I inch. Lakes, ponds, and stagnant water. P. a/5i(s.^'^ 



{e) Shell convex above with a central depression, concave 

 below, brownish-horn coloured, smooth, glossy ; whorls five to 

 six, more convex than in P. albus ; mouth nearly circular ; suture 

 well defined; umbilicus large. Diameter 2 lines. Ponds, 

 marshes, and lakes. Very local. P. parvusP {— P. glaber.) 



(/) Shell concave above^ flat below, or the reverse, thick, horn- 

 coloured ; whorls five to six, rounder and with the keel not so 

 well pronounced as in P. albus ; mouth roundish ; peristome often 

 white ribbed; umbilicus wide, shallow. Diameter, | inch. 

 Shallow and stagnant waters, sluggish streams. Common. 



P. spirorbis.'^'^ 



[g) Shell very flat, concave above, flat below, thin, glossy, 

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

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

 compressed ; umbilicus large, shallow. Diameter f ths inch. 

 Shallow and stagnant waters. Moderately common. 



P. vortex:-^ 



-° V. crista, transverse ridges more pronounced, periphery deeply notched 

 or crested by them. 



'-' V. Draparnaudi^ more closely striate in line of growth ; periphery dis- 

 tinctly keeled ; umbilicus larger. (This species is often found covered with 

 dirt, and requires to be well cleaned in order to show its characters ) 



-- V. coj/ipressa, more concave below, depressed i» the centre only on the 

 upper side, whorls rounder and not inci-easing so quickly. 



-^ V. albida, white ; v. ecariiiata, smaller, light green, keelless, one whorl 

 less than usual. 



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

 nearly in the middle of periphery. 



