MK. CLARK. 251 



Mr. Clark, a clever naturalist, considers with Mr. 

 Hancock that the powerfully armed ventral portion 

 of the mantle of the closed boring acephala is fully 

 adequate to rub down their habitations, and that the 

 theories of mechanical boring, solvents, and ciliary 

 currents, are so utterly worthless and incapable of 

 producing the effects assigned to them, as not to be 

 worth dwelling upon for one moment. Mr. Clark, 

 therefore, comes to the conclusion that ' the foot is 

 the true and sole terebrating agent in the Pholas.' 

 This 'fact' he considers to be ' incontestably proved,' 

 for the following reason, viz., because he had dis- 

 covered specimens of this bivalve with the foot en- 

 tirely obliterated, which phenomenon, Mr. Clark 

 states, is caused by the animal having arrived at its 

 full growth, at which period the terebrating functions 

 cease ; and as ' nature never permanently retains 

 what is superfluous/ the foot is supposed gradually 

 to wither away, and finally disappear. 



This, I suspect, is another 'fancy' theory. Al- 

 though I have excavated hundreds of Pholades, some 

 of giant-like proportions, it has never been my lot to 

 witness the foot otherwise than in a healthy and fully 

 developed condition. 



Another writer, having no opportunity of viewing 

 the living animal, does not consider it difficult to 

 imagine the Pholas c licking a hole' with its foot, 

 from the fact that he (Mr. Sowerby) managed to 

 make 'a sensible impression' upon a piece of kitchen 



