200 MULTIVALVES. 



were ranked by Linnaeus amongst the multivalves, 

 but by modern conchologists they are generally 

 placed with the bivalves . The two principal valves 

 connected by a hinge, associate them closely to 

 the latter, and the small testaceous plates around 

 the hinge may be considered as mere appendages, 

 for they do not act the part of valves. The Pho- 

 las appears the connecting link between the classes 

 of bivalves and multivalves : but so gradual are 

 the steps in the scale of nature, that it is often 

 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 (puXeot; 

 (pholeos) a hiding place. The largest species, 

 and the finest specimens are most frequently 

 found in chalk, which being the softest of calca- 

 reous substances, admits of a more easy and ra- 

 pid progress to the animal than the indurated 

 stones in which they are sometimes discovered. 

 How they effect their lodgment, is still a subject 

 of doubt and inquiry. Many have supposed that a 

 continued rotatory * motion would in time pro- 

 duce such a cavity as they inhabit, but as it is ex- 

 actly 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 



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



