CLASSES, ORDERS, ETC; 67 



(a) Shell quoit-shaped, thin, glossy, yellowish-horn coloured, 

 more or less reddish; whorls four to five, the outer whorl being 

 without septa as in P /ineatus, and very large in proportion to the 

 rest, covering one half of the preceding whorl, and bluntly keeled 

 in the middle of its periphery ; spire not so much sunk as in P. 

 lineatus ; umbilicus small, shallow. Diameter 2\ lines. Ponds 

 and ditches on aquatic plants. P. fontanus^ 



(b) 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 T Vth inch. 

 Ponds and ditches on aquatic plants. P. nauiileus^ 



(c) 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 

 |th inch. Lakes, ponds, and stagnant water. P. albus.^ 



(d) 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 ^th inch. Ponds, 

 marshes, and lakes. Very local. P. pawns?* ( P. glaber). 



(e) 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 Jth inch. Shallow 

 and stagnant waters, sluggish streams. Common. P. spirorbis.^ 



(/) Shell very flat, concave above, flat below, thin, glossy, 



19 v. albidci, white ; v. minor, smaller. 



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

 or crested by them. 



21 v. draparncmdi, 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.) 



22 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. 



