AXCYLUS. 71 



Ancylus lacustris, Mutt. Patella oblonga, Light. 

 Velletia lacustris, Gray. 



This little mollusk may be readily distinguished from 

 the former species by being more elongated and de- 

 pressed ; having the apex less recurved,, and to the left 

 instead of the right ; it is also thinner and more trans- 

 parent. The apex is placed near the centre of the 

 shell. Length a quarter of an inch ; breadth the ninth 

 of an inch. Animal dark grey, dextral, similar to flum- 

 atilis, except in the dentition. 



Hal. Generally distributed in England, but some- 

 what local, it prefers stagnant ponds, and is generally 

 found adhering to grass and aquatic plants. 



AURICULIDJE. 



CONOVOLUS. Lamark. 



Shell Spiral, oval ; aperture ovate, pyriform, entire, toothed 

 within ; outer lip simple or slightly reflected, more or 

 less thickened. JN~o operculum. 



Animal With two triangular and ringed tentacles, having 

 eyes at their inner bases. 



C. BIDENTATUS. Montagu. PI. VII, fig. 43. 

 Shell ovate, ventricose, smooth ; pillar with two folds. 

 Voluta bidentata, Mont. 



The shells belonging to this species vary in shape, 

 but are uniformly of a yellowish horn colour above and 

 white underneath the petiostraca. Whorls six or seven, 

 small and moderately convex. It has two teeth or folds 

 on the inner lip, as the name implies, which readily 

 distinguish it from any other of the genus. The spire 

 of the shell is sometimes eroded. Not quite a quarter 

 of an inch in length, breadth a line and a half. 



var. ALBUS, (PI. VII. fig. 44,) narrower, slenderer, 

 and more fragile ; spire more produced. 



Animal white, with two very flat, short, setose tenta- 

 cula?, with eyes at their inner bases. 



