214 LAKES AND RIVERS, 



P. vortex (Linne) has a thin compressed shell, with 

 6 or 8 whorls, gradually enlarging ; the length is 0-05; 

 the breadth 0-3. 



The variety compressus is still flatter, and the shell 

 thinner. It lays from ten to twelve eggs. P. carinatus 

 (Miiller) has the body a deep reddish-brown, with a 

 yellow tint underneath, marked with fine black spots. 

 The shell is compressed, concave above, and slightly 

 convex below ; the epidermis is thin ; the length is 

 OT, and the breadth 0-5. The variety disdformis is 

 flatter and thinner, of a yellowish colour ; the last 

 whorl is larger in proportion to the others. It in- 

 habits marshy and stagnant pools in the home and 

 eastern counties, as well as in several northern and 

 southern counties. P. complanatus (Linne) has 

 the shell concave above, and slightly convex below, 

 solid and opaque, yellowish horn colour, with a tinge 

 of brown; length, 0-125, breadth, o '6. The variety 

 rhomboides, the shell smaller, rather more convex 

 above, and deeply umbellicated below. Variety albidus, 

 shell whitish, or colourless. It inhabits marshes, ponds, 

 and standing waters throughout the Kingdom, highly 

 polished; the epidermis is very thin. The whorls 

 are 6 or 7 in number, convex, but slightly compressed. 

 It is found from the Moray Frith to the Channel 

 Isles, but is local. It is an Upper Tertiary fossil. 

 P. corneus (Linne) has the body dark-red, or 

 nearly black, greyish beneath, with black and gray 

 specks on the upper part ; the tentacles are long and 

 curved with blunt tips ; the eyes a moderate size. The 

 ' shell is rather concave above and nearly flat below, 

 of a whitish horn colour, with a reddish-brown tinge, 



