Dear Dr. Kirkwood,
the organs which enable Elasmobranchs  to perceive electrical fields are
called "ampulleae of Lorenzini". They are located on the underside of the
snout of an Elasmobranch. Their evolutive origin is most likely the lateral
line.

More to read:
a) Montgomery, J.C. (1988) Sensory physiology. In Physiology of
Elasmobranch Fishes, ed. by. T.J. Shuttleworth, pp.79-98. Springer-Verlag,
Berlin.
b) Kalmijn, A.J. (1978) Electric and magnetic sensory world of sharks,
skates and rays, in Sensory Biology of Sharks Skates and Rays, Hodgson,
E.S. and Mathewson, R.F., Eds., US Dept. of the Navy, Office of Naval
Research, Arlington, Va., pp. 507-528.

Hope that helps, Jascha Minow.

Jascha B. Minow
Free University of Berlin
Germany
e-mail: [log in to unmask]