46 NERITID^E. 



other vessels to replace with new portions the part 

 which has been removed. 



The apices of the spires of these shells are some- 

 times eroded ; those are more so which live in stag- 

 nant or nearly stagnant waters. The late Mr. 

 Sowerby (Min. Conch, iv. 49.) supposed that this 

 was produced by " some acid developed during the 

 fermentation of vegetable matter in marshes or at 

 the bottoms of the rivers." Others, who were not 

 aware how the animals walked, have said that this 

 erosion of the apex was produced by the animal 

 rubbing it against the ground in progression; ex- 

 plaining also the erosion of the umbones of the 

 Uniones in the same manner. 



Dr. Shuttleworth believes that the erosion of the 

 apex of some of the fluviatile Neritince is caused by 

 the animal's filing off the apices by its teeth, the 

 mark of the teeth being left on the surface of the 

 erosion. This is particularly the case with the 

 specimens which live in streams in granitic coun- 

 tries. And he suspects the animals supply them- 

 selves with the calcareous matter to form their shell 

 from this source, the body of the NeritincR being 

 withdrawn from the top of the shell as the shell is 

 enlarged* in size. 



10. 1. NERiTiNA^Mw'atf/zs. River Neritine. Shell 

 convex, dilated, tessellate, with variously co- 

 loured spots; spire short, lateral, (t. 10. f. 124.) 



Neritina fluviatilis. Lamarck, vi. ii. p. 188. ; Flem. B. ./4.321. ; 

 Tnrton, Man. ed. 1. 138. Neritina europaea. Leach, Syn. 

 Moll. 181. Nerita fluviatilis. Linn. S. Nat. 1253.; MMer, 



