MOLLUSC A. 



217 



its inner surface. The oyster, lima, and scallop 

 {Asipho7iia) have but one adductor muscle, and are 



Fig. 69. — Tbe Sea Mussel, Mytilus edulis, showing the foot giving rise to 

 byssus threads, and the mantle fringed with gills. 



called Mono my a ; the sea-mussels {Asiphonia) have 



Fig. 70. — Shell of the common Coclcle, Cavdlum editle. 



two, and because the posterior is larger than the 

 anterior they are called Heteromya ; while the Isomya, 



