32 



natural doubt exists as to whether there are two dis- 

 tinct species. The typical difference 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. 



Yar. I. rhombea. Smaller, more solid, more convex 

 above and concave below. Only faintly carinated. 



Yar. II. albida. Whitish. 



10. P. CORNEUS (horn-coloured). 



Yery 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. 



