CLASSES, ORDERS, ETC. 6/ 



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

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

 whorls three, the outer whorl bluntly and indistinctly keeled on 

 its periphery, and strongly marked with transverse ridges in line 

 of growth ; mouth oval ; umbilicus large. Diameter -^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. albus. 21 



(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. parvus.^ (= 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 -fths inch. 

 Shallow and stagnant waters. Moderately common. 



P. vortex.^ 



" v. crista, transverse ridges more pronounced, periphery deeply notched 

 or crested by them. 



- 1 v. Drapaniaudi, 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.) 



' a v. compressa, more concave below, depressed in the centre only on the 

 upper side, whorls rounder and not increasing so quickly. 



23 v. albida, white ; v. ecarinata, smaller, light green, keelless, one whorl 

 less than usual. 



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

 nearly in the middle of periphery. 



