BIVALVE MOLLUSCANS. 247 



their crypts, the shell revolving upon it as upon 

 an axis. 



Mr. Osier, in the memoir before alluded to, 

 states that the pholads can be observed to 

 burrow only in the young state : and that they 

 are found completely buried when so minute as 

 to be almost invisible. The guiding hand of 

 Providence excites them from their very birth 

 to fix themselves by their pointed foot, to erect 

 their shells, and giving them a partial rotatory 

 motion which employs the valves alternately, 

 thus to enlarge their habitation, and this almost 

 constantly, since the rapidity of their growth, for 

 the first few weeks, compels them to act perse- 

 veringly in effecting that object, for the raspings 

 of its crypt would clog the animal if they were 

 left in it. When the siphon is distended with 

 water, the animal, closing the orifices of its tubes, 

 suddenly retracts them : thus a jet of water is 

 produced which is prolonged by the gradual 

 shutting of the valves, and clears the shell and 

 the crypt. 



There is another family of bivalves which 

 bores the rocks, the species of which are in- 

 structed by their Maker, to accomplish their 

 object by a very different process. I allude to 

 Lamarck's family of Stone-eaters. 1 This family 

 contains only two genera, removed from Venus, 



1 Les Lithophages. 



