PHOLAS STONE PIERCER. 195 



principal valves connected by a hinge, associate 

 them closely to the latter, while the small testa- 

 ceous plates around the hinge may be considered 

 as mere appendages, for they do not act the part 

 of valves. The Pholas appears the connecting 

 link between the classes of bivalves and multi- 

 valves. So gradual are the steps by which the 

 scale of nature proceeds, that it is of\en difficult 

 to ascertain precisely the line which separates its 

 great divisions. 



The Pholades are all borers, perforating wood, 

 clay, limestone, and sometimes burying them- 

 selves in the sand, forming a retreat in which they 

 pass their lives. The name marks their peculiar 

 propensity, being derived from ^wAws (pholeos) a 

 hiding place. The largest species, and the finest 

 specimens are most frequently found in chalk, 

 which being the softest of calcareous substances, 

 admits of a more easy and rapid progress to the 

 animal than the indurated stones in which they 

 are sometimes discovered. How they effect 

 their lodgement, is still a subject of doubt and 

 inquiry. Many have supposed that a continued 

 rotatory* motion would in time produce such a 

 cavity as they inhabit, but as it is exactly fitted 

 to their size, it cannot be easily imagined that 

 the animal could perform this motion within it. 

 Another circumstance opposed to this opinion 

 is, that the mollusca whose shells are perfectly 

 smooth, are capable of producing the same exca- 

 vation as those which have a surface as rough 



* Rotatory, like a wheel, from the Latin rota, a wheel. 



