594 MOLLUSCA 



Suborder 5. MYACEA. 



Shell either regular or irregular, not pearly, with or without hinge 

 teeth; ligament internal or external; siphons long; pallial sinus present; 

 2 adductor muscles; mantle closed below with an opening for the small 

 foot: 5 families, all marine; all burrowing in sand, mud, or rock. 



Key to the families of Myacea here described: 



Ci Shell long and narrow, margins usually parallel 2. SoLENlDAE 



Oa Shell not long and narrow. 



6i Left valve with shelf-like process for ligament 1. Myidae 



6a No such process 3. Saxicavidae 



Family 1. MYIDAE. 



Shell rather thick, inequivalve, gaping at one or both ends ; ligament 

 internal, attached in left valve to a prominent shelf-like process; hinge 

 teeth variable; periostracum wrinkled; pallial sinus large; siphons par- 

 tially or wholly retractile, united: 250 species. 



Key to the genera of Myidae here described: 



Ci Hinge toothless 1. Mya 



Ca Hinge with a tooth on each valve 2. Corbula 



1. Mya L. Shell oblong, gaping behind and more or less irregular; 

 siphons very long and partially retractile, covered with periostracum; 

 hinge toothless; foot small, with byssus in youth: 3 species, 2 on the 

 Atlantic and Pacific coasts. 



M. arenaria L. Soft-shell clam. Long clam (Fig. 936). Shell 

 ovate, white, covered with a brownish periostracum; length 10 cm.; 



Fig. 936 Fig. 937 



Fig. 936 — Mya arenaria (Verrill). 1, foot ; 2, mantle ; 3, cloacal siphon ; 4, branchial 

 siphon. Fig, 937 — Vorbula contracta (Gould). 



height 6.5 em. ; umbo nearer anterior end : Arctic Sea to Cape Hatteras, 

 between tide lines and in shallow water in mud flats and under stones; 

 San Francisco; Europe; a common article of food. 



M. truncata L. Shell truncated behind; periostracum thick and 

 extended posteriorly, forming a tube 15 cm. long; length 7 cm.; height 

 4 cm.; width 3 cm.: circumpolar, south to Cape Cod and Puget Sound; 

 not common ; Europe, where it is more plentiful than M. arenaria. 



