46 
NERITIDJE. 
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 NeritincB 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 Neritincc being 
withdrawn from the top of the shell as the shell is 
enlarged in size. 
10. 1. ^^mumAfluviatilis. River Neritine. — Shell 
convex, dilated, tessellate, with variously co- 
loured spots ; spire short, lateral, (t. 10. f. 124.) 
Neritlna fluviatills. Lamarck^ vl. ii. p. 188. ; Flem. ; 
Turton^ Man. 1. 138. — Neritina europsea. Leach., Syn. 
Moll. 181. — Neritafluviatilis. Linn. S. Nat. 1253.; Mullei\ 
