spiny softshell: river turtle of western United States with a warty shell; prefers quiet water
soft-shelled turtle: voracious aquatic turtle with a flat flexible shell covered by a leathery skin; can inflict painful bites
astatic coils: an arrangement of coils used in sensitive electrical instruments; the coils are arranged to give zero resultant external magnetic field when a current passes through them and to have zero electromotive force induced in them by an external magnetic field