32 LIMN HID. 
natural doubt exists as to whether there are two dis- 
tinct species. The typical differenee is in the position 
of their keels, that of P. carinatus being in the centre 
of the outer margin, while that of P. complanatus is 
much lower—in fact, touching a flat surface when 
placed upon it. The typical P. complanatus is thicker 
and larger than P. carinatus, but it is difficult to 
decide upon half the individuals met with. The well- 
marked complanatus, however, is more often met with 
than the well-marked carinatus, and the former species 
is therefore allowed to be the commoner. 
Var. I. rhombea. Smaller, more solid, more convex 
above and concave below. Only faintly carinated. 
Var. II. albida. Whitish. 
10. P. cérneus (horn-coloured). 
Very tumid, spire sunk, opaque, dark horn-colour, lighter 
below; mouth nearly circular ; whorls five to six. 
This species is unmistakable, both from its size and 
shape. It is far the largest of our English Planorbis. 
When young it is covered with spiral rows of small 
hairs. It is very abundant in many districts in canals, 
streams, and ponds, but it is considered local. I have 
some specimens with four marks showing stages of 
growth, which, if they are formed only once a season, 
imply that these individuals were in their fifth year. 
