PHOLAS. 25 



very wide, and the dorsal edge is turned back over the 

 umboes : such is the state of shell, which is far from co- 

 vering the animal, while it continues to grow and to bore, 

 the latter operation implying some sort of motion. Whether 

 this motion be rotatory or not, we shall not now stay to in- 

 quire ; but it certainly must cease when the adult stage is 

 attained, because at that period the shell begins to be shaped 

 so as to fit the hole exactly. A transformation in general 

 appearance then takes place; the reflected dorsal edge 

 before spoken of becomes thickened and expanded, and a 

 pair of joints or articulations are added, which stand for ru- 

 dimentary accessories; the wide front hiatus is closed by a 

 thin, paper-like inflated extension of the edges, and, still more 

 remarkable, a portion of the hollow at the upper end of the 

 burrow is filled up by a completely formed, protruding, cup- 

 like appendage, which is like the commencement of a tube 

 at the thinner end of the shell. 



The Pholas papjracea is not very easily procured in a 

 perfect state. Mr. Hanley observes that it "must be con- 

 sidered not only a very local shell, but one difficult to pro- 

 cure even at most of the spots from whence only it can be 

 obtained. It is met with at very low tides imbedded in 

 reddish sandstone at Exmouth, Teignmouth, Torquay, and 



