FAMILY LYMNiEACEA. 135 



Planorbis corneus. 



Genus I. PLANORBIS, Guettard. 



Animal ; body slender, carrying a discoidly convoluted tubular skell, 

 bead obtuse, tentacles long and bristle-like, with tbe eyes at 

 tbeir base, mantle not reflected over tbe sbell, foot small, narrow. 



Shell ; discoid, sometimes very tbinly compressed, lens-shaped, 

 apex sunk in tbe nucleus of tbe coil, whorls varying from three 

 in number to seven, smooth or striated, sometimes keeled at 

 the periphery. 



Of the Ammonite-skated Lymnceacea associated in this genus we 

 have eleven species in Britain, nearly aU that are known in Europe; 

 and among them is one, the subject of our vignette, pre-eminent in 

 size, the largest form of Planorbis in the world. The body of this 

 mollusk is remarkably slim and tapering, enveloped by a mantle 

 which secretes a rotary coding tubular shell of horny or glassy sub- 

 stance increasing wonderfully slowly in diameter. The head of the 

 animal is stout and obtusely proboscis-shaped, the tentacles slen- 

 der and bristle-like, with the eyes small and black at their inner 

 base, and the foot, as might be expected where the aperture of the 

 shell is so contracted, is small, narrow, and very flexible. The 

 most slenderly convoluted species are P. carinatus, complanatus, 

 vortex, and spirorbis, the two first encircled at the periphery by a 

 fine marginal keel ; a sixth species, P. contortus, is chiefly remark- 

 able for the closeness with which the whorls are pressed one upon 

 another; only one species has any definite sculpture, P. albus ; 



