248 



SYSTEMATIC ZOOLOGY 



water more than sixty seconds. The animals are allowed to die, 

 the soft parts washed away, and then the pearls are sought for ; 

 some, the finest, are free, others are attached to the shell and 

 must be chipped off. Pearls are probably always secreted about 

 some object, often a bit of sand, and are formed by many other 

 bivalves, though of not so fine a quality as by the pearl oyster. 

 Of the other salt-water mussels, aside from those which form 

 a chief article of food in many parts of Europe, there are the 

 boring mussels, in which the body is greatly elongated, often hav- 

 ing a very small shell. Some of these make cavities in stone 

 as well as in wood (Figs. 256 and 257), such as Lithodomus 



Fig. 257. Boring mussel in a fragment of limestone. (Photographed from a specimen 



obtained at Bermuda, by the author.) 



dactylus. The borings in the columns of the temple of Serapis 

 at Pozzuoli near Naples show the geologist the degree of sub- 

 mergence of these columns beneath the sea at one time. The 

 fresh-water mussels which live in our ponds and rivers and 

 brooks carry their young about in a sort of brood pouch in the 

 gills. One species, Margaritana margaritifer, found in Europe 

 and the United States, produces pearls of considerable value 

 and often pink in color. 



Of the Siphoniata or Lamellibranchia with the edges of the 

 mantle fused posteriorly into a siphon, the edible species are 

 the most familiar. The cockles, having" greatly arched, swollen 

 shells, are largely eaten in England. The common clam, J/r<? 

 arenaria, is a very familiar article of food in our Eastern states. 



