256 POPULAR BRITISH CONCHOLOGY. 



nceus. That of P. liypnorum is oblong and conical ; while 



that of P.fontinalis is oval, with a large aperture. 



Planorbis. — The tentacula of this genus are subulate ; 



the head stout and broad; the foot short and truncated. 



The shells are coiled up, so as to form flattened discs ; they 



have been likened to Ammonites, commonly called ^^petrified 



snakes." 



P. corneus : the shell, which, like the greater part of the 

 species, is horny in texture and colour, is the thickest 

 species, having the whorls few, deep, and squarish; it 

 sometimes occurs nearly an inch in diameter and half 

 an inch in the depth of the whorls. The animal is 

 black, and gives out a purple fluid when irritated. 



P. alhus has a very small white shell, the spire of which, 

 although flat, is not sunk; the aperture is wide and 

 oblique ; surface microscopically striated. P. glaher 

 has the shell somewhat similarly shaped and smooth. 



P. nautileus has sharp, oblique ridges on the whorls of its 

 very minute, wliite shell. 



P.' carinatus and P. marginatus have very flat shells, with a 

 keel round the circumference of the disc, which is about 

 two-thirds of an inch in diameter. The former is more 

 polished and more depressed than the latter species. 



