MOLLUSC 1 A 135 



while in others it is used for rapid burrowing'. The 

 oyster and other species, however, possess a foot only 

 in their young stages, while in the mussel (Mytilus) 

 and related genera, the foot contains a gland which 

 secretes silk-like fibers which constitute a byssus or 

 "anchor". These may be used for temporary or 

 permanent attachment. In a few species the foot is 

 used as a rasping or boring organ and may drill a 

 hole through other shells (a3 does Urosalpinx, Thais, 

 etc.) or may even penetrate hard rock (as does Litho- 

 domuSj Pkolas, etc.). 



Most bivalves possess two tubes or siphons; 

 through one the water passes into the animal and 

 through the other it is discharged. In some species 

 the siphons are very long and conspicuous (as 31 t/<i, 

 Pholas), while in others they are very inconspicuous 

 and are contained within the shells. 



The group Pelecypoda contains various species 

 of considerable economic importance. While the 1 

 Hard Shell and Soft Shell Clam, the oyster and the 

 scallop, are the only bivalves frequently eaten in 

 our region, many other species are also edible and 

 are used as food in other parts of the world. 



While the species mentioned in this book are all 

 inhabitants of salt or brackish water, many bivalves 

 live in freshwater streams and lakes. 



Solemya velum Say 



PLATE XV. Fig. 6 



These shells are very thin and fragile, about an 

 inch long and Vo inch wide and covered by a corne- 

 ous epidermis of deep chestnut color. Radiating 

 lines are prominent. Known from Nova Scotia to 



