52 MOLLUSCA. 
of our workmen’s files in this, that the surface keeps 
itself always in a proper state of roughness for 
trituration. This is done by an organic law, which 
causes the crystals to be constantly shed, and. as 
constantly renewed.* 
All the borers above alluded to are Bivalves, 
and I know of no other Mollusk which can pro- 
LIMPET, 
perly be classed with them. A common Gastero- 
pod, however, the familiar Limpet (Patella vulgata), 
excavates the rock on which it lives to the extent 
of making a depression, more or less deep, exactly 
corresponding to the shape and size of the margin 
of its shell. When one removes a Limpet from 
its firm adhesion and finds a hollow beneath it, 
evidently made to contain its body, one is ready to 
conclude that the animal is a permanent tenant of 
the spot, never moving from it; and when we learn 
that the food of the Limpet consists of sea-weeds, 
we wonder how it is possible that a stationary 
animal can find vegetable food. But the truth is, 
* Ann, and Mag. N. H. Oct. 1848. 
