114 MALACOZOA. GASTEROPODA. PULMOBRANCHIATA. 



foot small, oval or elliptical, depressed ; orifice of the pul- 

 monary cavity on tlie coUar, near the anus, on the left 

 side, as are those of the separated genital organs. 



Shell orbicular, flat, coiled nearly in the same plane, 

 generally delicate and more or less transparent ; with 

 a roundish, sublunate, or squarish aperture, somewhat 

 encroached upon by the convexity of the preceding turn, 

 and having a thin border, and a simple cavity. The 

 shell is dextral, as several observers have proved, not 

 sinistral, as many have alleged. This will be apparent 

 on placing a Helix, a Zonites, and a Planorbis beside 

 each other. In Helix in its natural position, the oblique 

 aperture of the shell has its upper and anterior margin 

 more advanced, and the spire projecting to the right. In 

 Zonites, the same circumstances are observed, the spire 

 being little prominent, or quite flat. Planorbis is similar, 

 but with the spire flat or more generally sunk. 



The species live in stiU and running water, generally 

 on plants. 



\. Planorbis Vortex. Angular-mouthed Flat Coil-Shell. 



Shell extremely depressed, very thin, pellucid, glossy, 

 slightly and regularly concave above and beneath, with six, or 

 sometimes seven, gradually increasing volutions, which are con- 

 vex above, rather flattened beneath, and transversely rugoso- 

 striate, the outer rounded but sloping so as to form a rather sharp 

 sometimes obtuse keel on the lower margin of the disk ; mouth 

 oblique, roundish-elliptical, somewhat rhombic. Diameter 

 four-twelfths, height less than half a twelfth. 



This species, as it presents itself with us, has the keel much 

 less acute, and the lower surface of the turns much less flat- 

 tened, than in specimens from England and the South of Scot- 

 land. In young shells the keel is often acute, but in old indi- 

 viduals it is often obtusely angular, and even rounded toward 

 the mouth, wdiich in that case is not rhombic, but roundish, or 

 broadly elliptical. 



The animal, in moving, presents an apparatus which at first 

 sight appears scarcely adequate to the task of dragging forward 

 so large a mass as the shell and body. It consists of an ellip- 

 tical, depressed or flattened foot, a twelfth and a third in 

 length, little more than half a twelfth in breadth, of a choco- 



